The physiology of the circulation in plants, in the lower animals, and in man : being a course of lectures delivered at the Surgeons' Hall to the president, fellows, etc. of the Royal College of Surgeons of Edinburgh, in the summer of 1872

Front matter

EX BIBLIOTHECA

bo

THE

Preface

In the present volume I have endeavoured to give a comprehensive account of the Circulation and the apparatus by which it is effected in Plants, in the Lower Animals, and in Man. I have discussed the subject in its totality, in the hope that those parts of the circulation which appeared obscure in the higher animals might be simplified by comparing them with similar parts in the lower animals and in plants. This mode of treatment involves details, but these, I trust, will be found also to add additional interest. I have introduced a large number of drawings in place of the diagrams originally employed in illustrating the Course, in order to efface in some measure the traces of lecturing. I have not, however, found time to recast or alter the Lectures in any way, so that, with the exception of a few verbal corrections, they appear in their original form. This circumstance will not, I trust, detract from their usefulness.

My best thanks are due to my friend Mr. Archibald Constable for having relieved me of the labour of reading the proofs for press.

Royal College of Surgeons of Edinburgh, April, 1874.

Introduction

PAGE

Epitome of the History of the Circulation 4

Definition of the Circulation . 7

CIRCULATION IN PLANTS.

Two Different Systems in Plants 10

Two Principal Currents in Plants — Proof that the Saps ascend and descend 1 1 Endosmose and Exosmose as Adjuncts of the Circulation . . . .15

The Vessels of Plants — their Function 17

Points of Resemblance between the Vessels of Plants and Animals . . 20

Respiration of Plants and Animals 23

Cells of Plants — their Nature and Function . . . . . .24

The Intracellular Circulation in Plants 28

The Lactiferous Circulation in Plants 41

The Forces which produce the Circulation in Plants 42

The Vessels of Plants in Summer form Syphons 49

Intracellular Movements in Plants — Probable Explanation of . . .51

Absorbents of Plants . . 56

The Vessels of Plants form a Double System of Syphon Tubes — Analogy

between Leaves and Roots 58

The Effects produced on the Circulation by the Swaying of Plants in the

Wind. — Mr. Spencer's Views 61

Epitome of the Forces engaged in the Circulation of Plants .... 64

Organic Forces a Modification of Inorganic Ones . . . 74

Motion a Condition of Matter 77

Circulation in Metals ........... 79

CIRCULATION IN ANIMALS (Invertebrata).

Symmetry of Form in the Organs of Circulation and the Body generally . 80

Respiration and Assimilation connected with the Circulation . . A 8a

Ciliary Currents 82

V1 CONTENTS.

Rhythmic Movements— Analogy between Involuntary and Voluntary Move- PAGE

ments g.

Circulation in the Leech contrasted with that in Plants .... 89

How the Circulation connects the Organic and Inorganic Kingdoms . . 91

Digestion and Circulation q2

Cilia— their Form and Function g4

Undefined Forces of the Circulation — Slowing and Quickening of the

Circulation, etc. <j6

Circulation in the Star-fish joj

Circulation in the Spider 103

Circulation in the Insect 103

Function performed by the Valves of Vessels 106

Circulation in the Lobster. — Position of the Respiratory Apparatus in the

Lobster, Fish, etc 108

Circulation in the Brachiopoda and Gasteropoda no

Circulation in the Cuttle-fish in

CIRCULATION IN ANIMALS (Vertebrata.)

Circulation in the Fish . in

Circulation in the Aquatic Reptiles 117

Circulation in the Frog 118

Circulation in the Lacerta Ocellata 121

Circulation in the, Python, Crocodile, etc. — Presence of Septum Ventriculorum 122

FOETAL CIRCULATION.

Points of Analogy between the Circulation in Reptiles and that of the Human

Foetus . . 122

Circulation in the Body of the Foetus 123

Circulation in the Placenta . 128

Distribution of the Great Vessels in Reptiles, Birds, and Mammals . .159

Valves in the Heart of the Bird 161

Respiration and Circulation in Birds

The Air-Sacs of Birds, etc 163

Circulation in the Mammal . ■ .

Auricles, Ventricles, Valves, Septa, etc. of Mammalian Heart . . .167

Right Heart of Mammal l(>8

The Blood urged on by a Wave-Movement; Heart and Vessels Open and

Close in Parts l69

Valves of Right Heart of Mammal . . . • • • • • I75

Safety- Valve Action of Tricuspid

Left Heart and Valves of Mammal l77

Circulation in the Head, Liver, and Erectile Tissues 17s

The Lymphatic and Capillary Vessels of Animals 180

Structure of the Capillaries, Small Arteries, and Veins : Vessels close and

elongate, and open apd shorten 182

Elastic and Vital Properties of Vessels . 184

CONTENTS. Vll

PACE

Structure of the large Arteries and Veins i§7

Structure of the Heart of the Mammal : the Arrangement of the Muscular

Fibres in the Auricles and Ventricles 1 9°

Analog}' between the Muscular Arrangements and Movements of the Hollow

Viscera (Heart, Stomach, Bladder, Uterus, etc.) and those of the Trunk

and Extremities of Vertebrates 201

Structure and Properties of Voluntary and Involuntary Muscles . . . 208

Involuntary Muscle 208

Voluntary Muscle 208

Mixed Muscles •. . . . • • .211

Muscles can act in Two Directions ; Sarcous Elements of Muscle ; Peculiar

Movements of . . . ..211

Muscular Motion as bearing on the Functions performed by the Pleaft,

Blood-vessels, Thorax, Extremities, etc 211

How Muscular Fibres open and close the Vessels, the Compartments of the

Heart, the Stomach, Bladder, Thorax, Abdomen, etc. . . . .224 Analogy between the Movements of the Thorax, Abdomen, and Heart . 233 The Movements of the Mammalian Heart, interrupted and yet continuous.

— How the Heart rests 237

Size of the Cavities of the Heart (Mammal) . . . . ... . 240

Impulse of the Heart (Mammal) 241

THE VALVES OF THE VASCULAR SYSTEM.

Venous Valves — their Structure, etc 243

The Venous Valves in Action 249

Arterial or Semilunar Valves, their Structure, etc. 251

The Arterial or Semilunar Valves in Action 259

Semilunar and other Valves of the Fish, Reptile, Bird, etc 259

Intricate Structure of the Mitral and Tricuspid Valves in Mammalia; Relations of the Chordae Tendineae to the Valves and to the Musculi

Papillares 2(56

The Mitral and Tricuspid Valves of the Mammal in Action . . . .273

The Mechanical and Vital Theories of the Action of the Mitral and Tricuspid

Valves considered ........ 2*76

The Sounds of the Heart, to what owing 284

THE GANGLIA AND NERVES OF THE HEART, AND THEIR CONNEXION WITH THE CEREBRO-SPINAL AND SYMPATHETIC SYSTEMS IN MAMMALIA.

The Cervical Cardiac Branches 2^g

The Thoracic Cardiac Branches 2 g

The Superior Cervical Ganglia 2 y

The Middle and Inferior Cervical Ganglia ] 2Q_

The Upper Cardiac Nerve

The Middle Cardiac Nerve . ' 297

The Inferior Cardiac Nerve . L via

CONTENTS.

PACE

Plexuses formed on the Roots of the Pulmonary Artery and Aorta in the Calf 301

Plexus formed on the Anterior Coronary Vessels of the Calf . . . 302

Plexus formed on the Coronary Sinus^of the Calf 302

Distribution of the Nerves on the Surface and in the Substance of the Auricles

of the Calf 303

Distribution of the Nerves on the Surface and in the Substance of the

Ventricles of the Calf 303

Distribution of the Nerves on the Right Ventricle 306

Distribution of the Nerves on the Left Ventricle 307

Distribution of the Nerves in the Human Heart 309

Microscopic Appearances presented by the Ganglia found on the Coronary

Sinus and Cardiac Vessels 310

Why does the Heart act Rhythmically? 313

Proofs that the Heart may act independently of the Nerves . . . .316 Proofs that the Movements of the Heart are regulated and co-ordinated by

the Nerves 318

The Theory of Irritability as bearing on the Action of the Heart considered 321

Concluding Remarks 329

THE PHYSIOLOGY OF THE CIRCULATION

IN PLANTS, IN THE LOWER ANIMALS, AND IN MAN.

INTRODUCTION.

Mr. President and Gentlemen, — The circulation of the blood forms such an important factor in the animal economy that I feel I need offer no apology for making it the subject of the present lectures. The heart, its nerves, blood-vessels, and valves, supply a theme of unusual interest not only to the anatomist and physiologist, but also to the practitioner, especially since the introduction of the stethoscope. In order to give an intelligible account of the circulation, and the apparatus by which it is effected in man, it will be necessary to avail myself of whatever collateral information is within reach. This will lead me to speak at more or less length of the so-called circulation in plants — of the circulation in the lower animals, and in the foetus.

By adopting this method I hope to be able to lead up to the complex circulation as it exists in man, by a series of steps, which, when taken together, will form a sort of royal road to the goal at which we would ultimately arrive. The time devoted to the journey will not be lost if I succeed in placing before you the links (fearfully and wonderfully made) of a chain, on the integrity of which life itself may be said to depend. In some cases we shall find only one link of the chain present ; in others, two ; in others, three ; and

it r B so on until we arrive at a degree of differentiation and completeness which, while it commands the reverence, must elicit the admiration of every one who reflects. In the human circulation nearly all the links are present, and it is only by a knowledge of these, as they exist in the lower grades of life, that we can hope to put them in their proper places, when we come to generalise. In the plant and medusa there is no trace either of a heart or pulsatile vessel. In the insect the heart is absent, but a pulsatile vessel makes its appearance. In the fish a heart, consisting of two cavities, is found. In the serpent those cavities are increased to three ; and in birds and mammals to four.

The number of the cavities depends to a certain extent on the nature of the respiration, and this in its turn modifies the temperature of the blood. In the fish breathing is effected, and the blood aerated, by the gills ; one cavity (the auricle) receiving the blood from the system ; the other (the ventricle) forcing it directly into the gills, and, through them, indirectly into the' system. In the serpent breathing is effected, and the blood aerated, by the aid of lungs ; two cavities (the auricles) receiving the blood from the lungs and from the . system ; one cavity (the ventricle) forcing it through the lungs and through the system. In the reptile the circulation is of a. mixed character, the arterial and venous blood blending in the ventricle of the heart. In the bird and mammal the heart consists of four cavities — two auricles and two ventricles ; the auricles receiving the blood from the system and from the lungs ; the ventricles propelling the blood through the lungs and through the system. This object is attained by a cross circulation — the right auricle receiving venous or dark blood, the left auricle arterial or bright red blood ; the right ventricle propelling venous or dark blood, the left one arterial or bright blood. The differentiation observable in the cavities of the heart is preceded by a similar differentiation in the channels and vessels through which the nutritious juices flow ; the elaboration of the heart and vessels necessitating the presence of valves, which vary in number according to the complexity of the circulatory apparatus. ' In the lowest organised plants, such as the fungi, algae, etc., and in the lower classes of animals, as the polypi, actiniae, and a great part of

PHYSIOLOGY OF THE CIRCULATION.

the intestinal worms, the nutritious materials are transmitted through their substance, without any distinct canals or tubes ; while in the higher classes of plants, and in the medusae, etc., among animals, vessels are present, but these are unprovided with any pulsatory cavities. In the articulated animals the vessels are still without any pulsatory cavities, but to make up for the deficiency the dorsal vessel itself has a distinct movement of contraction and relaxation V

With regard to the valves, it may be stated that they are only found in well-formed vessels, and at the orifices of canals or cavities having a definite structure. They are so placed that they compel the circulating fluid always to move in the same direction. They are consequently not found in plants, for in these, as will be shown presently, the nutritious saps at one period of the year flow from below upwards, and at another, from above downwards. A cross circulation, i. e. a circulation across the stem, and a circulation within the cells of the plant, are likewise to be made out. The valves vary in number and in the complexity of their structure. In the veins they usually consist of from one to four segments. In the arterjes, as a rule, they consist of three segments2. In the heart they may vary from two to three, and to these as many intermediate segments may be added. The number, however, may greatly exceed this, for in the bulbus arteriosus of the American devil-fish (cephalopterus giorna) we find as many as thirty-six segments. The valves may be placed within comparatively unyielding structures, as the fibrous rings found at the beginning of the pulmonary artery and aorta of the mammal ; or they may be placed within yielding and actively moving structures, as the bulbus arteriosus of the fish, and the base of the ventricles of the bird and mammal. In the former case the structure of the segments is comparatively simple, and their action to a great extent mechanical ; in the latter case the structure of the segments is more complex, the segments

1 Cyc. of Anat. and Phys., art. ' Heart.' By Dr. John Reid, p. 577.

2 The valves of the arteries in the mammalian adult are confined to the origins of the pulmonary artery and aorta. They are termed semilunar from their shape. It happens occasionally that only two segments are present, the number increasing at times to four. In such cases a segment may be absorbed or divided into two by disease. It ought to be observed that rudimentary valves are found in the umbilical arteries. These are, strictly speaking, foetal structures.

B 2

PHYSIOLOGY OF THE CIRCULATION.

being provided with tendinous chords which vary in length, strength, and direction. The tendinous chords are necessary to restrain the action of the segments within certain limits, and to co-ordinate the movements of the segments with the movements of the structures within which they are placed — the bulbus arteriosus and ventricle opening and closing alternately. The forces engaged in carrying on the circulation increase in direct proportion to the number and complexity of the hearts, vessels, and valves, and the number and variety of the tissues to be nourished. It is with a view to reducing those structures and forces to their simplest expression, that I propose to treat the subject of the circulation comparatively. What is true of the particular is necessarily true of the general ; and if we succeed in comprehending a simple structure, and in following a simple action, the knowledge acquired will greatly assist us in comprehending complex structures and combined actions. It is comparatively an easy matter to understand a purely mechanical act. Here the forces and resistances can be appreciated with something like mathematical accuracy. It is comparatively a very difficult matter to understand a vito-mechanical act, for in this case we are never exactly sure what is vital and what vito-mechanical. This is especially true of the circulation, where we have on some occasions to deal with rigid tubes and cavities, such as exist in plants, and which are incapable of receiving an impulse from without ; while in others, we have to deal with flexible elastic tubes and cavities, such as are found in animals, and which are not only capable of receiving an impulse, but of storing up the impulse or power communicated, and of expending it as required.

If to the foregoing we add that the action of muscles, — which, in animals, are the chief motors in the circulation, — is comparatively unknown, we conclude a list of difficulties which it will require time, patience, and perseverance to overcome. These difficulties were present in force to the ancients, as a brief resume of the history of the circulation will show.

Epitome of the History of the Circulation. — We are in the habit, and very properly, of attributing the discovery of the circulation to the illustrious Harvey. It ought, however, not to be forgotten that there were many pioneers in this field before Harvey. The Chinese, for example, believed that the circulation of the vital heat and radical humours commenced at three o'clock in the morning, reached the lungs in the course of the day, and terminated in the liver at the end of twenty-four hours. . This was a very vague and visionary notion of the circulation certainly; still, it embodied the idea of fluids circulating within the body. The first rational attempt at. unravelling this great mystery was made in the time of Hippocrates and Aristotle. It consisted of a description, apparently from dissection, of the principal blood-vessels. Galen, towards the end of the second century, described the course of the blood-vessels in many of the lower animals, and appears to have known the structure and uses of the foramen ovale in the foetus, and that the arteries and veins anastomosed. He was also cognizant of the fact that the arteries contained blood, and described them as arising from the heart ; the veins, in his opinion, arising from the liver. He thought that the blood passed through the septum of the heart ; and this is actually true of the heart of the serpent. Neither Hippocrates, Aristotle, nor Galen, however, was aware that the blood circulated, i.e. started from one point and returned to the same point after making the circuit of the body. Galen believed that the blood simply oscillated ; and it is a curious circumstance that, in plants and some of the lower animals, this is what actually occurs.

Mr. Herbert Spencer, for instance, states that in plants the saps ascend and descend in the same vessels, and that their movements are interrupted at irregular intervals. Johannes M tiller, in like manner, affirms that in the leech the blood-vessels on one side of the body contract and force the blood into those of the other and opposite side, and that these in turn contract and force it back again, the blood being made to oscillate transversely across the animal. The celebrated Vesalius examined the subject of the circulation afresh in 1542. He pointed out the difference between the arteries and veins, and showed that the veins and the heart contained valves. He also explained that the arterial pulse depended upon the systole of the heart. Here was a decided advance. Servetus, that martyr to science,1 had, as early as 1 53 1, actually

1 Servetus fell a victim to religious persecution, and was cruelly burned alive at Geneva in 1553.

PHYSIOLOGY OF THE CIRCULATION.

described a pulmonic circulation. He says, Whereas, in the adult, the vital spirit (blood) cannot pass from the right auricle into the left, because of the imperforate nature of the auricular septum, it must go through the lungs, where it is changed, i.e. undergoes a transformation by coming in contact with the vital spirit which resides in the air, after which it returns to the heart. Servetus expressed his belief that the pulmonary artery and vein had some other function than that of nourishing the lungs — an inference deduced from their comparatively large size.

Caesalpinus, in 1583, described the pulmonic circulation more carefully than Servetus had done, and showed that he had some knowledge of a double circulation. Nor did the foetal circulation, with its several peculiarities, escape the lynx eyes of the anatomists of the sixteenth century. Galen, as has been pointed out, had a knowledge of the foramen ovale. He had also a partial knowledge of the ductus arteriosus. The ductus arteriosus was carefully described by Fallopius, Aranzii, and Vesalius. The last observer likewise discovered the ductus venosus, which was figured by Fabricius and Eustachius. . It was reserved however for the immortal Harvey to place the coping-stone on the magnificent edifice of the circulation, and to this philosopher we owe our knowledge of the circulation in its entirety, and as at present accepted. Harvey was the apt pupil of a celebrated master. He studied under the famous Fabricius of Padua, for the protracted period of six years, viz. from 1596 to 1602. Fabricius, although entirely ignorant of the circulation as subsequently developed and explained by Harvey, had, fortunately for Harvey, an intimate knowledge of the valves of the veins, of which he published an account in 1603. From Fabricius then the illustrious Harvey derived much of that information which enabled him to astonish the world by his brilliant discovery. Harvey, on his return to England, instituted a series of experiments with a view to determine the exact nature and uses of the venous valves. The inquiry was laborious and long ; and it was not till the year 1619, i.e. seventeen years after he left Padua, that he ventured to teach the doctrine of the double circulation publicly. Other nine years elapsed before the Exercitatio Anatomica de Motu

PHYSIOLOGY OF THE CIRCULATION.

Cordis et Sanguinis in AnimaKbus saw the light. This celebrated work was consequently published in 1628, and, curiously enough, not in England but in Frankfort. It therefore took Harvey some twenty-six years to mature and perfect his views.

Harvey left little to be done, but that little fell to the lot of a worthy successor, the renowned Malpighi, who, in 1661, by the aid of the microscope, discovered the capillary circulation, and the presence of blood-globules within the vessels. Harvey's views, thoroughly matured, and sound in warp and woof as they were, met with a determined opposition, and it was only after many an unseemly conflict between really great men, that the blood was allowed to flow quietly in the double channel which he had taken such pains to discover and describe. There is little chance of its changing its course now ; and the task which I set myself in the present lectures is to point out the course which the blood and other nutritious juices pursue, as they meander about in the organic world, now free, now confined ; now checked by the presence of valves, now hurried forward by the pressure of vessels, hearts, and other forces, to which I shall have occasion frequently to allude as the subject becomes more complicated.

Definition of the Circulation. — The term circulation, in the present day, is employed in a double sense. In its wider signification it embraces the course of the nutritious juices through plants and the lower orders of animals ; in its more limited signification, and as applied to man and the higher orders of animated beings, it indicates the course of the blood from the heart to the capillaries, and from these back again to the heart. The word circulation literally means a flowing round — a going and returning ; and it is well to bear the original meaning in mind, as we shall find that a single circle aptly represents the circulation in most of the lower animals ; a circle, with one or more accessory loops, representing the circulation in the higher ones. In man, the chief circle represents the systemic circulation — two accessory loops representing the pulmonic and portal circulation. The accessory loops may be increased indefinitely to meet the requirements of any particular case.

PHYSIOLOGY OF THE CIRCULATION.

CIRCULATION IN PLANTS.

In plants the circulation is in its most rudimentary form ; in fact it is incomplete as a circulation, i.e. as a continuous flow of nutritious juices in a circle. My meaning will be obvious when I remind you, that in trees the sap flows steadily upwards in spring, and steadily downwards in autumn. The chain is, as it were, broken at both ends. There is, in addition to the upward and downward currents, a certain amount of transfusion, i. e. of cross currents running in the direction of the breadth of the stem ; but the transfusion referred to is trifling when compared with the two principal, and, as regards time, interrupted currents 1. In order to comprehend the true nature of this interrupted and, so to speak, disjointed circulation, it' is necessary to remember that trees have a season of activity and a season of repose ; that they increase in an upward direction by means of shoots, in a downward direction by means of roots, and laterally by branches2. In other words, they increase in every direction; and this holds true of growing animals as well as growing plants. The shoots, by their upward growth, tend to draw up the sap ; the roots, by their downward growth, to draw it down ; and the branches, by their lateral growth, to draw it transversely. Here, then, are the materials for an interrupted or disjointed circulation with a certain degree of oneness about it. Nutrition is a principal factor in the process. The

1 ' The nutrient fluids in plants follow certain directions according to the structure and arrangement of the tissues, the locality of the sources of nutrition and of growth, or other actions ; and as regards the elaborated fluid, the movement may be — (i) from the place of formation to that of consumption ; or (2) to the stem, cells, or reservoirs; or (3) from the latter to the place of consumption.' — Henfrey's Botany, as edited by Dr. Masters, F.R.S., p. 570.

2 ' The sap will flow to the several parts according to their respective degrees of activity — to the leaves while light and heat enable them to discharge the,ir functions, and back to the twigs, branches, stem, and roots when these become active and the leaves inactive, or when their activity dominates over that of the leaves. And this distribution of nutriment, varying with the varying activities of the parts, is just such a distribution as we know must be required to keep up the organic balance.' — Principles of Biology, by Herbert Spencer, vol. i. p. 557.

rilVSIOLOGY OF THE CIRCULATION.

idea of a- plant or tree increasing in an upward and downward direction and laterally at one and the same time, is consistent with fact. When a seed grows, it extends itself into the ground and into the air ; i. e. it spreads from a centre in an upward and outward direction, and in a downward and outward direction \ (Figs. 1-3.)

Fig. 1. Fig.*. FiS-3-

Fig. 1. — 'A seedling: dicotyledonous plant with an ascending and a descending* axis.' — Henfrey. Fig. 2. — ' Pandanus odoratissimus, the screw-pine, with adventitious roots supporting the trunk.' — Henfrey.

Fig. 3. — ' Rhizophora mangle, the mangrove-tree, supported as it were upon piles by its numerous roots, which raise up the stem. The plant grows at the muddy mouths of rivers in warm climates.' — Balfour.

If, bearing this fact in mind, you imagine that as the tree grows the central point from which it had its being moves upwards (it must do this if it is to maintain its central position with reference to the tree as a whole), then we are forced to conclude that the stem of the tree itself is sending off processes in an upward and downward direction, and likewise laterally. That the central or germinal point recedes in an upward direction as the stem grows is rendered almost certain by the researches of Henfrey, who has shown that the root in growing exercises an upward pressure as well as a downward one ; and that if the upper part of the root be relieved from its load- of superimposed earth, and the weight of the plant balanced, the root, in virtue of its elongation alone, will cause the whole mass of the plant to rise bodily upwards. This is particularly well seen in the mangrove-tree and screw-pine,

1 The hydrogastrum, one of the algae, consists of a single cell, but this cell is so differentiated as to simulate a perfect plant, with roots, stem, bud, and fruit.

PHYSIOLOGY ON THE CIRCULATION.

where the stems are raised completely out of the ground, and supported as it were upon piles. (Figs. 3, 3.)

Two Different Systems in Plants.— The embryo, according to Petit Thouars and Gaudichaud, consists of two portions, a caulmary and radicular ; the one having a tendency to ascend, the other to descend. These portions, which may be taken to represent different systems, have different sets of cells and vessels ; the ascending system in the dicotyledons being connected with the medullary sheath, and passing into buds and leaves ; the descending system, which is situated between the sheath and the bark, being connected with the woody tissue

Fig. 4-

Fig. 5-

Fig. 6.

Fig. 4. ' Slip or cutting of root of Maclura, showing a number of buds, c, from which proceed radicular fibres, a, which are interposed between the bark, b, and the fibres of the old wood. The young fibres are traced to the buds, and, in their progress downwards, tiiey remain distinct. This example is brought forward by Gaudichaud as illustrating his vertical theory of wood formation.' — Balfour.

Fig. 5. ' Truncated stem of a dracaena after maceration, showing the tracheae, t. I, of the ascending system of the stem and branch. The radicular system of the old stem,./; is seen in the form of fibres, and the radicular woody bundles, r, of the branch are disposed in a grasping manner over those of the old stem. The fibres, according to Petit Thouars and Gaudichaud, come from the bases of the leaves, and belong to the descending system.' — Balfour.