Lessons in elementary botany : the part of systematic botany based upon material left in manuscript by the late Professor Henslow

Front matter

LESSONS

IN

E L E M E NTABY B 0 T A N Y,

Preface To First Edition

It was the intention of the late Professor Hen slow to publish a small volume of Lessons on Botany, in illustration of the methods which he employed with such eminent success in his class at the University of Cambridge, and also in his parish school at Hitcham, in Suffolk.'*

At the time of his lamented decease he had collected a considerable amount of material for this work, hut it was not left in a sufficiently forward state to send to the printer without addition and modification.

In July of the present year, Professor Henslow’s manuscripts intended for the work were placed in my hands by Dr. Hooker and the Rev. George Ilenslow, with full permission to make what use I thought proper of his material in the prepara-

* The “ Leisure Hour ” for 1862 (page 676) contains an account of Professor Henslow’s method of teaching Botany in Village Schools, by the Rev. George Henslow. See also Professor Henslow’s pamphlet, “ Illustrations U be employed in Practical Lessons on Botany, adapted to Beginners,” published by Messrs. Chapman & llall, 1858. Piicc 6d.

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tion of an inexpensive book, designed to teach the Elements of Botany on Professor Henslow’s plan of selected Types, and by the use of “ Schedules.”

Peeling that there might be good service in a book of this kind, which, instead' of interfering with the excellent text-books of Botany already published, might serve rather as introductory to them, and suffice, with constant reference to the plants themselves, to insure to the diligent learner a sound foundation for more advanced Botanical studies, I determined to prepare these “ Lessons in Elementary Botany,” basing the systematic portion (Part II.) upon the Type Lessons, which formed the largest and most valuable part cf Professor Henslow’s manuscripts.

As no sufficient introduction to Systematic Botany had been prepared by Professor Henslow, I have drawn up a few chapters, embracing the elements of Structural and Physiological Botany, treated in as simple a way as appeared to me consistent with practical usefulness. These elementary chapters it is desirable beginners should master before undertaking the methodical study of the Ordinal Types. In working up the latter, I have made the most free use of Professor Henslow’s materials, frequently quoting entire passages from his manuscript. I have, however, omitted much which Professor Henslow had

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introduced, with a view to show the important ‘instructional value” of Botany as an “educational weapon,” in the opportunity which Botanical Lessons afford the teacher of introducing to his class a large amount of useful information upon topics of the most varied kind. Thus I find a lesson in pneumatics apropos of pop-guns, made of the stem of the Elder ; an account of soapmaking, and the method of ascertaining specific gravities, in connexion with the Olive Order, represented in Britain by the Ash ; how to analyse a Potato, under the Bittersweets ; an account of wonderful bells, under the Bell-flowers ; of glass-making, under the Saltworts, formerly burnt for the sake of their alkaline salts ; of brewing, under Hops ; and the like. I have omitted such matter because it would require more space than could be afforded in a book of small cost, to be used by private learners as well as by teachers ; and also because the variety of topics w7hich a teacher may thus introduce, and which his own intelligence may suggest, is infinite.

Professor Henslow’s plan of employing “ schedules,” designed to direct the attention of the pupil at once to the most important points of structure, I have adopted 'in my own class with much advantage. Their use should be commenced as soon as the first six chapters are

PREFACE.

viii

thoroughly acquired. Any printer would strike off a few hundreds of blank “schedules” at a small cost, but the necessary form may be easily drawn with pen or pencil by the student himself. I strongly recommend their use, both by private students and in colleges and schools.

Most of the excellent woodcuts employed in this work were drawn by Professor Henslow’s daughter, Mrs. Barnard, of Cheltenham, from the admirable Sheet Diagrams designed by Professor Ifenslow, and executed by Mr. Pitch, for the Committee of Council on Education. They have been liberally placed at tbo disposal of the publishers by the Rev. George ifenslow.

Kew. September

LESSONS IN BOTANY.

Chapter I. A Buttercup Is Examined

1. The Root. — Its form and general structure : it pene-

trates the soil ; is colourless ; irregularly branched ; destitute of leaves ; and its extremities are sheathed.

2. The Stem. — It ascends ; is coloured ; bears leaves

and branches at definite points ; the extremities are not sheathed, but give off, successively, rudiments of leaves.

3. Foliage-leaves. — They . are borne by the stem only.

“Radical” and “cauline” leaves : thin, coloured green, consisting of an horizontally expanded blade with, or without, a petiole.

4. Flowers consist of leaves. The peduncle. Suppres-

sion of internodes in flowers. The receptacle of the flower.

5. The sepals ; forming the calyx.

6. . The petals ; forming the corolla.

7. The stamens ; bearing anthers which contain pollen-

grains. The stamens are hypogynous.

8. The carpels; each containing an ovule. The stigma.

The style. The carpels, collectively, constitute an apocarpous pistil.

9. The carpels persist after the other parts of the flower

fall away. The fruit ; seed ; embryo and albumen.

10. A summary of the parts examined.

GATHER, first of all, a specimen of the Common Buttercup;

There are at least three different plants included under this name, almost equally common in meadows and pastures over all England, and most of Scotland, but as they nearly resemble each other it does not signify which is taken. In gathering the specimen take it up carefully, so that the root may be uninjured.

Proceed now to examine your buttercup.

1. Observe the Koot, noting in what respects it differs from the parts which grow above ground. It consists of numerous fibres, about the thickness of small whip-cord, tapering at their extremities and giving off irregularly many thread-like fibrils. It is destitute of the green colouring of the stem and foliage, being pale or nearly white : it bears neither buds nor leaves, and its branches, from their direction, appear to have avoided the light.

If you can find the tip of one of the root-fibres uninj ured, cut it off, and examine it minutely with your magnifying glass. In ease you have not the means of examining . it

with a higher magnifier, you

J /--■ -! will find figure 1 a sufficiently

correct representation of it, rjo. 1. Longitudinal sec- , . . , , . , . ,,

tion through the ex- divided through the middle

oFmSt£cu^0tSage- aTld magnified many times, nified. — gp. growing The point which I want you root-fibre^' sheath' r*' particularly to note is this, The extremity of the fibre is covered by a closely fitting sheath, protecting the actual growing point, which is hidden immediately within the end of the sheath, to which it is directly joined. This protecting

A BUTTERCUP.

sheath is being constantly renewed, at its inner side, by the “ growing point,” so that as the outer layers become worn or withered, by forcing a way through the soil and. pebbles, they are constantly replaced by inner layers which take their turn, replace them, and then die ; to be in like manner replaced by fresh inner layers derived from the “growing point,” so long as the root continues to live.

You find, then, that the root avoids the light ; that it is pale or nearly white ; that its fibres give olf, irregularly, numerous delicate thread-like branches (fibrils) ; that it is destitute of buds and leaves, and that the tips of the root are protected by cellular sheaths.

2. Now examine the Stem.

You observe, at once, that the stem rises directly from the ground, seeking, rather than avoiding, the light. In one kind of buttercup^ growing in moist soils, there are two sorts of stem on the same plant ; one a creeping stem, which has the power of giving off roots freely at its joints, and which, spreading along the ground, multiplies the plant by forming offsets, and the other an erect stem bearing flowers. It is the flower-bearing stein we are speaking of just now.

Excepting the lowest thickened portion more or less buried in the soil, the stem is coloured green, and not being woody we may speak of it as herbaceous. It bears several foliage-leaves arranged on different sides of the stem. The lower ones- spring in a tuft from its base, and have long stalks. The upper foliage-leaves are without stalks, and arranged singly on the steni|

EXAMINATION OP although sometimes they are so deeply divided as to look as though they were in threes.

If we examine the growing point of a young stem under a magnifying glass, carefully dissecting away the leaves which surround it, we shall find that to the very apex it continues to give off, successively, minute lateral prominences, whicii are the rudiments of leaves. Both foliage and flower-leaves originate upon the stem in the same way, hut they soon become different, both in their arrangement, form, texture, and colour. The stem never terminates in a cellular sheath like that which protects the tips of the root.

The branches spring from points where foliage-leaves are given off from the stem ; each branch occupying the angle (called the axil of the leaf) which the leaf makes with the stem.

At the extremity of the principal stem of your fully-grown buttercup, you find a tuft of coloured leaves forming a flower. All the branches, also, end in flowers or flower-buds.

The stem, we find, ascends ; is coloured green, and is herbaceous in texture ; it bears foliage-leaves and ends in a flower.

3. Foliage-leaves. — I use the term foliage-leaves at present simply in order to avoid confusion with the leaves of which flowers are composed. It is not necessary you should always call .them so, hut it is necessary that you thoroughly understand that, speaking generally, whatever is borne by the stem and its branches is a leaf of some kind , whether it be green, as are foliage-leaves, or coloured, as are flower-leaves.

We have already remarked that the lowest leaves have long stalks. As they seem to spring

A BUTTERCUP.

from the root they may be called radical leaves. They really spring from a portion of the stem, which is thickened and more or less buried underground, giving off root-fibres below and radicalleaves from above. This portion of the stem is called the stock.

The point on the stem from which any leaf is given off is called a node; the space between two nodes is called an internode. It is owing to the non-development of the internodes of the stock that the leaves which it bears appear to be given off in a tuft.

Each radical leaf consists of stalk and blade ; the stalk supporting the blade is called a petiole. The base of the petiole, observe, is more or less sheathing, and the blade is much divided into deep segments , which again are further more or less lobed. The upper leaves, obviously springing from the stem, may be described as cauline. Being destitute of petioles, they are termed sessile. The same word is used of any part of a plant to denote the absence of a stalk, whether that stalk be a petiole (which is the stalk of a foliage-leaf only) or not. All the foliage-leaves have the blade spread out more or less horizontally, and they are all coloured a deep green. They may be hairy , or nearly glabrous , that is, destitute of hairs.

We find, then, the foliage- leaves to be borne by, and around, the stem ; they are thin, coloured green, and consist of petiole and blade, or of blade only ; the blade being spread out horizontally. We now come to the examination of the

4. Flowers, and as the leaves of which these consist are smaller than the foliage-leaves, and some of them, in the buttercup, very small indeed.

it will be necessary that you be very careful in your observations, making sure that you thoroughly understand every stage of your progress.

The upper part of the stem serves as a stalk to the flower. Flower-stalks are distinguished as 'peduncles. The peduncle of the buttercup may be either round or furrowed, according to the kind which you have gathered. Before proceeding to dissect (to separate carefully into its pieces) a flower, select one that has but just opened, and which has lost none of the parts which it possessed while still a bud; that is, before it expanded.

Observe, first, that all the coloured leaves which form the flower are apparently arranged

FiG. 2. Vertical section of flower of Buttercup, showing the parts of the flower inserted upon a conical receptacle.

upon the very summit of the stem. The internodes of the stem which separate the upper foliage-leaves cease, or are suppressed , in the flower, so that all the parts are in close juxtaposition. This is characteristic of flowers. The top of the flower-bearing stem, from which the

A BUTTERCUP.

flower- leaves collectively spring, is called the receptacle, or floral receptacle.

5. Proceed next to note that there are, in the flowerj five separate outer leaves arranged in a whorl ; small, coloured yellow, and either spreading or sharply curved hack, according to the kind of buttercup you are examining. These leaves are each entire — that is, without the lobes or divisions of the foliage-leaves. They fall off early, and hence may be described as deciduous. These live outer leawes of the dower are singly termed calyx-leaves or Sepals ; together they form the Calyx of the flower. The sepals being free, that is, separate from each other, and from the other parts of the flower, the calyx is poly-sepalous .*

6. Immediately inside the calyx are five, rather larger, separate, nearly sessile, brightly coloured leaves. These also are arranged in a whorl, and they are singly placed opposite to the intervals between the sepals ; not opposite to the sepals themselves. Like the sepals they are deciduous. Singly, they are the corolla-leaves or Petals ; the five together form the Corolla of the flower. The petals being free the corolla is polypetalous ; being equal in size and form, it is also regular.

7. In the examination of the rest of the flower much nicety is required. Having stripped off the sepals and petals singly, and laid them aside, proceed to the third series of flower-leaves.

* The prefix poly- (no\vs, ipany) is used when applied to sepals and petals , to^denote that the sepals or petals aie free rather than that they are actually many in number. Usually there are not more than three, four, or five sepals or petals in a flower.

Fig. 3. Stamen of Buttercup.

These are very different in form and structure from both sepals and petals, though, like them, they are all free ; that is, distinct from, and independent of, each other. They each consist of a lower stalk-like || H portion, hearing an upper somewhat \\\ y thickened, oblong, and grooved head.

This stalk is termed the filament; the oblong head which it supports, the anther ; and these together constitute a staminal leaf or Stamen. The stamens of the buttercup are shorter than the petals. As they are numerous, and, as we have just observed, free, they are said to he polyandrous.

The anthers we must examine more close] y. We have already observed that there is a groove up the back (outer side), and another, less distinct, along the face (inner side). These grooves divide the anther into two lobes, right and left. If the anther he ripe, each of these lobes will split open near the outer edge, allowing certain fine powdery granules which it contains to be easily removed by insects or otherwise. These granules are essential to the flower as well as to the stamen, so we must carefully examine them under a microscope. Tig 4 shows them highly magnified.

We find that they are distinct globular cells with minutely granular The fine powder is the pollen , and each globular cell is a pollen-grain.

Remove all the stamens, noting the minute,-

Fig. 4. Pollen-grains of Buttercup (magnified).

contents.

A BUTTERCUP.

closely-packed, and spirally arranged scars which remain after you have picked, them off. You find the stamens, like the sepals and petals, inserted directly upon the floral-receptacle. When this is the case they are technically described as hypogynous.

8. You have now left a small head, consisting of numerous distinct, but very closely-packed, sessile leaves, extremely unlike either sepals, petals, or stamens. Separate one of them, and try to cut it through lengthwise. Fig. 5 shows one thus treated. It is hollow, containing a single, pale-coloured body, attached to the base of the cavity.

This is the rudiment of a future seed, and it is termed, in its present stage, an ovule. The hollow leaf which contains and protects the tender ovule is called a carpellary leaf or Carpel.

Examine the outside of any one of these carpels with a magnifier, and you must observe that the upper portion is somewhat pointed, ending in a loose, cellular tip. This cellular surface is termed the stigma. It is invariably present. The lower portion of the carpel, containing the ovule, is the ovary. The stigma is very shortly stalked in tho buttercup. In many plants the stalk of the stigma is of considerable length, sometimes several inches ; whether long or short, however, it is called a style. If there be no style, the stigma is sessile.

Fro. 5. Vertical section of a carpel of Buttercup, containing one inverted ovule.

The structure of all the carpels of the buttercup is identical : they each contain one ovule, and are each provided with a stigma, and a very short style.

The carpels, taken together, constitute the pistil ; they are the fourth and last series of the ilower-leaves, and occupy the centre of the flower and, consequently, the extremity of the stem. As the parts of the pistil in the buttercup are wholly free from the calyx, the pistil is termed superior, and as the carpels which compose it are free from each other the pistil is also apocarpous.

9. Gather now another specimen of the buttercup : one in a more advanced state, with the sepals, petals, and stamens of some of its flowers all fallen away, and only the heads of carpels (pistils) remaining.

The pistil here is passing into fruit. The carpels remain distinct from each other as at first, but they have enlarged and hardened ; the stigmatic apex has dried up, so that the top of ,, _ , each carpel is simply pointed or

section of a fruit- slightly hooked. Opening any

ButtercSStai^- riPe CarPel> a single seed wil1 be ing one seed. found filling its cavity. If we

cut one of these seeds right through lengthwise, we shall find that it consists of a very thin (membranous) shell, the testa, enclosing a solid, uniform and horny substance, called the albumen ; near the base of which, very careful observation will detect a minute body, the embryo or germ of a future plant. Fig. 7 shows such a section of the seed and the position

A BUTTERCUP.

Pig. 7. Longitudinal section of a seed of Buttercup, showing the minute • embryo at the \>ase of the albumen.

of the embryo in the albumen. Sow, if you like, a few of the fruit-carpels ; the carpel will decay away, and the seed, under favourable circumstances, will germinate and grow up into a buttercup plant.

10. You ought now to have a good idea of a buttercup, so far as its general- structure is concerned.

But before we proceed to future chapters, in which we shall inquire into the relation of the various parts of the plant to each other, into the office or function which each is intended to perform, and the relation which the organs of other plants bear to those of our buttercup, let us recapitulate the different parts which we have observed thus far. If there be any part which has not been clearly made out, make a point of understanding it before proceeding further.

We have, first, a Eoot, which descends into the soil, gives off fibrils irregularly, and is pale-coloured ; the fibres have their extremities sheathed, and, as they do not give off the rudiments of leaves, the root is leafless. It is at first directly continuous with, and appears to pass into, the stem, but, in plants which are fully grown, the original root is supplemented by other root-fibres which are given off from the lower part of the stem.

The Stem ascends, bears foliage-leaves, from the axils of the upper of which spring branches ; it is coloured green more or less, and terminates in the head of coloured leaves forming the flower.

Eoot and stem, therefore, we find opposed to each other in the directions which they respectively take, as well as in several points of structure. Together they may be regarded as constituting the axis of the plant ; the root being the descending, the stem the ascending portion of the axis. Upon the ascending axis all the leaves, both foliage and flower-leaves, are arranged.

Leaves. — These, we have found, are of five different kinds. First are the radical and cauline

(1.) Foliage-leaves, called simply leaves.* Then come the flowers, terminating the stem and its branches, consisting of —

(2.) Calyx-leaves , called sepals.

(3.) Corolla-leaves, called petals.

(4.) Staminal-leaves , called stamens.

(5.) Carpellary- leaves, called carpels.

* In many plants we may distinguish two modifications of the leaf below the flower-leaves, besides the green foliage-leaves, hut as it is not important to distinguish these in the buttercup we shall pass them by at present.

A BUTTERCUP.

Chapter II. Examination Of A Buttercup Continued

1 . The buttercup fades. Why ? Experiment shows that

it is because water is withheld.

2. The root an absorbing organ.

3. Water is exhaled from the leaves. Transpiration.

Absorption.

4. Other substances, besides water, ai’e absorbed. The

ash and inorganic constituents of plants.

5. The organic compounds of carbon, oxygen, hydrogen,

and nitrogen.

6. Ternary and quaternary compounds of these elements

are in contrast.

7. Source of carbon in carbonic acid.

8 Liberation of oxygen by plants under the influence of sun-light. Assimilation.

9. Source of nitrogen.

10. Processes of absorption, transpiration, assimilation, and respiration, performed by ‘organs of nutrition,’ viz. the root, stem, and leaves.

1.T3Y this time the specimen of buttercup has 13 probably faded; the leaves, now become soft and flaccid, are drooping, and the stem has lost much of its stiffness. How is this 1

If the specimen be not quite withered, plant it again in the soil, and cover it with a flower-pot, or, better and more easily done, put the root in water, and place the whole in a cool shaded place for a few hours. We shall anticipate matters and suppose that this has been already done, and that you tried the experiment in this way upon three distinct specimens of buttercup. No. 1 you left lying upon the table. No. 2 was placed with its root in water. No. 3 was hung upside down, with a flower or leaf in water, the roots in the air.

The general result of your experiment will be as follows : — After the lapse of, say, twenty-four hours or so, No. 1 will be, as we have already found, faded ; No. 2 will be nearly unaltered ; No. 3 will be partially faded, the parts out of water especially. Hence we may gather that water supplied to the specimens prevents them from fading, especially if it be supplied to the root. On the other hand, if water be withheld, they fade.

2. If we take now the faded specimen first described and put its root in water, and leave it for a few hours in a cool, shaded room, we shall probably find, unless it be irretrievably withered, that it freshens more or less, the leaves and stem becoming firmer and more nearly like their original state.

This experiment shows us, further, that water supplied to a fading buttercup enables it to recover.

Reflecting upon these experiments, we shall be led to the following conclusions : —

i. That water evaporates from the exposed surface of plants.

ii. That fresh supplies are taken into the plant by the root.

iii. That the stem serves to convey this watersupply from the root to the leaves.

A BUTTERCUP.

3. We may now try another simple experiment, devised by Professor Henslow, which shows that exposure to direct sunlight, rather than the dryness of the air, has to do with this evaporation of water from the leaves. Take six or eight of the largest, healthy, radical leaves of the buttercup you can find ; two tumblers filled to within an inch of the top with water, two empty, dry tumblers, and two pieces of card, each large enough to cover the mouth of a tumbler. In the middle of each card bore two or three small holes just wide enough to allow the petiole of a leaf to past shrough. Let the petioles hang sufficiently deep to dip into the water when the cards are put upon the tumblers containing it. Having arranged matters thus, turn the empty tumblers upside down, one over each card, so as to cover the blades of the leaves. Place one pair of tumblers in the sunshine, the other pair in a shady place. In five or ten minutes examine the inverted tumblers. That exposed to the sun you will find already lined with dew on its cool side, while that kept out of the sun is still nearly or quite clear. It is manifest, therefore, that evaporation from the leaves must be not only rapid, but considerable in amount when plants are exposed to the sun whether in a dry or moist atmosphere.

This exhalation of vapour from the surface of plants is termed transpiration. A correct understanding of the process explains how it is that plants growing in parlours are apt to become faded even when watered, because the taking up of water (termed absorption) by the roots cannot always keep pace with the transpiration from the leaves, owing to the rapid evaporation excited by dryness of the air. Since the specimen No. 3, experimented upon at the beginning of the present lesson, faded, notwithstanding the immersion of some of its leaves, it is clear that the root is the part which performs the office of absorption principally.

Every part of a plant or animal appropriated to a distinct purpose or function is termed an organ. Hence the root may be called the organ of absorption of the plant.

4. Now, not only is water absorbed by the root, but also various substances which are dissolved in the water. Hence we find, if we burn a plant carefully, that an ash remains, consisting of such of these substances as are not dissipated by' heat, which were absorbed in this way, and which had been made use of by the plant, or stored away in its tissues. Of the simple elements known to chemists, about twenty occur in the ash of plants ; many of these, however, in very minute quantities, and never all in the same plant. Sulphur, phosphorus, potash or soda, lime, and silex, are those most generally found.

5. But if we analyse an entire plant, and not the ash only, we shall find constantly present, besides the above, the elements carbon, oxygen, hydrogen, and nitrogen. And these elements are presont, there is reason to believe, in every organized being, whether plant or animal, in combinations peculiar to organized beings. Hence, they may be called the organic elements , in contradistinction to the (mineral) elements found in combinations which are not peculiar to organized ueings, and several of which remain in the ash

A BUTTERCUP.

of plants when burnt. The latter may be called the inorganic dements.

6. These four organic elements do not exist separately in the plant, but, as we have said, in combination. Thus the carbon is united with oxygen and hydrogen (the last two the elements of water), forming the basis of a series of compounds, called ternary compounds , because they are composed of three elements. The nitrogen occurs combined with the same three elements, forming a quaternary compound, or compound of four elements. And these two series of organic compounds stand in remarkable contrast to each other in the plant, both in respect of the structures in which they respectively take part, and of function, as we shall point out when we come to speak of the minute structure of plants.

7. We have already explained how water (oxygen and hydrogen) finds access to the plant, as well as certain mineral substances which may be held in solution by the water. With regard to the important element carbon, experiments clearly show that it is absorbed in combination with oxygen, as carbonic acid gas, which occurs as a constant constituent of the atmosphere ; and green leaves, under the influence of sunlight, possess the power of absorbing it directly from the air.

8. But the most remarkable circumstance attending this absorption of carbonic acid is the liberation of oxygen gas by the leaves, very nearly to the amount absorbed in combination with the carbon of the . carbonic acid gas. This liberation of oxygen is most easily shown by taking a few leaves which have been first soaked a day or two in water, so as to become saturated, and exposing them, plunged in water containing carbonic acid (as ordinary spring or pump water, in which it is always present), to direct sun-light. Minuto bubbles will be given off, under favourable circumstances, in a rapid and continuous stream. These bubbles consist of almost pure oxygen.