Nature and development of plants.

Front matter

¥ NATURE AND DEVELOPMENT OF PLANTS

BCURTIS:: SS

SIXTH EDITION—REVISED

Contents

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NATURE AND DEVELOPMENT OF PLANTS

Introduction

1. The Nature of the Plant.—It is a familiar fact that most plants have stems, leaves and roots and that these organs are variously modified. Why have such organs been developed? Why do they assume such varied forms and arrangements? How does it come about that the stem reaches up in the air and that the branches and leaves are arranged in a very definite order while the root penetrates the soil and grows toward moisture and other soil material? Isa plant, like an animal, conscious of its surroundings, seeking suitable foods and avoiding unfavorable conditions? It is evident to the casual observer that the plaat is sensitive to its surroundings; some plant organs bend toward a light that we recognize with difficulty or react to a touch that we cannot appreciate. Has the plant a nervous system and faculties like ourselves that enable it to adjust itself to its surroundings? To answer these questions it will be necessary to

Fic. 1. Diagram of a cell showing its three dimensions; mesh-work of granular cytoplasm which incloses colorless cell sap,.the vacuoles, and a denser dark body, the nucleus.

consider the nature of the living substance of the plant. The

plant body is composed of cells that are like boxes of living

matter having length, breadth, and thickness (Fig. 1). While

the cells assume various forms and perform different work or I

2 THE LIVING SUBSTANCE OQF®PLANTS functions, they all possess cell walls which contain during the life of the cell a substance termed protoplasm. This substance resembles somewhat the white of an egg, being viscid and rather tenacious. The protoplasm is not of uniform consistency. The larger portion of it is finely granular and is called the cytoplasm while a denser rounded part is termed the nucleus. Other bodies, plastids, also denser than the cytoplasm are of common occurrence. Such plastids as contain a green pigment, chlorophyll, are called chloroplastids or chloroplasts, and these prodticé the green j D i

Fic. 2. Structure of the cell: A, cell ditch-moss, Philotria; n, nucleus, v, vacuole, ch, chloroplast. B, cell of carrot; ch, yellowish chromoplasts, 1, nucleus. C, reddish chromoplasts from rose hip. D, cells of Begonia; s, starch grains, c, crystals of lime. £, leucoplast, /, of potato forming a starch grain.

color of the vegetation (Fig. 2, A). Still other plastids, the chromoplasts, contain red or yellow pigments that give the color to many fruits and flowers, as the tomato, rose-hip, squash, nasturtium, etc. (Fig. 2, B, C). Colorless plastids, leucoplasts, often

INTRODUCTION 2 occur in cells hidden from the light. One group of these leucoplasts forms the starch that appears in underground storage organs as in the potato (Fig. 2, EZ). Usually spaces or vacuoles, that appear as cavities also occur in the granular cytoplasm (Fig. 2, A,v). In reality they are filled with watery solutions of various substances, the so-called cell sap. Many other structures (Fig. 2, D) appear in the protoplasm to which attention will be called later. It is well to remember that these visible structures do not represent the real composition of the protoplasm. A variety of units beyond the range of visibility, grow, multiply, and build up the various structures which we recognize as constituting the protoplasm. The cell walls make up the body of the plant and give stability to its various organs but the living part of the cell is the protoplasm. This substance reaches through delicate pores in the cell wall to the protoplasm of adjacent cells so that all the living substance of the entire plant body is in contact and forms one united mass.

2. The Nature of the Living Substance of the Plant.—The protoplasm possesses most remarkable powers. It can absorb various fluids and gases, decompose them into simple elements, reunite them into foods or discharge from the cell such substances as are not required. Furthermore the protoplasm effects those changes termed growth by transforming the foods into the substances that compose the cell walls and other parts of the cell, as the protoplasm itself. How are these changes brought about? Every substance is composed of elements. Water consists of two elements, hydrogen and oxygen. The elements that compose any substance are held together with great energy, owing to their mutual attraction for one another. When two different substances are brought together, it may happen that the attraction of certain elements of one substance is greater for one or more of the elements of the other substance than for its own elements. The result is, that the elements will be torn away from their respective substances and united into new combinations. We say that a decomposition and a re-combination has been effected, ot, that a chemical reaction has occurred. We see an illustration of these chemical changes when iron-ore is heated with charcoal.

4 NATURE OF THE LIVING SUBSTANCE

The ore is composed of the elements iron and oxygen and the charcoal consists of carbon. In the presence of heat the carbon has a stronger attraction for the oxygen than the irori has. Con-_ sequently the oxygen is drawn away from the iron and unites with the carbon, forming a new combination of carbon and oxygen, and leaving the iron free. This decomposition and re-combination is only a part of the chemical change that takes place, but it will serve to illustrate the nature of many chemical reactions. The energy that was required to hold the oxygen and iron together begins to be set free as soon as the decomposition starts and it contributes to the rise of temperature during the reaction. The living substance of the plant is composed of a great variety of chemical substances that are very readily decomposed by gases and other substances which it absorbs. Light, — heat, gravitation and moisture are even more powerful in producing these changes. These substances and forces are often.

referred to as stimuli (sing. stimulus) because they start the chemical changes referred to above. So the various substances and forces in nature stimulate or cause chemical changes in the living substance of the plant and the energy thus set free is used by the protoplasm in the performance of its work. The cells of a seed have no power of their own to grow or perform any duty. It is not until a certain amount of heat, moisture, gases or other stimulating forces have acted upon the protoplasm or substances contained in it and so aroused chemical changes that it is furnished with the requisite energy to begin growth. Furthermore, these forces exert a very definite influence upon the protoplasm and cause it to accomplish very definite results. This is due to the fact that not all portions of the protoplasm of the plant body are equally influenced by, or we may say equally sensitive to heat, light, and other forces and consequently the various parts of the plants do not respond alike. The force of gravity, for example, acts upon some stems and the living substance is influenced by this stimulus so that a growth is aroused that bends the stem into an upright position. The protoplasm in the cells of the root, however, is so constituted that the stimulus of gravitycauses a growth that bends the root down into the soil. Light stimulates the mustard plant in such a way that the stem grows toward the light and the root away from it. So the living substance of the plant while forming one united whole varies in its sensitiveness in various parts of the plant.

In this respect the various parts may be compared to the receiving stations of a wireless telegraphy system. The receiving stations may have instruments sensitive to certain intensities of electrical currents and other currents do not affect them. So the various organs of the plant are sensitive, each in its own peculiar way, to certain stimuli. As a result of this adjustment the various parts of the’ plant, being attuned or sensitive to certain forces, are led or induced to grow so that they come into the most helpful and beneficial relation to these forces. Thus the leaves, branches, and roots are stimulated to grow and develop so that each part becomes properly related to light, moisture, gravity, etc. The broad blades of the majority of leaves are exposed to the direct rays of light. This arrangement is of the greatest benefit because they are using the light in the manufacture of foods. This position is assumed, however, because they are sensitive to light, gravitation and other forces which direct and cause the arrangement. This is the most noteworthy feature about the sensitiveness of the protoplasm. Forces stimulate to growth in such a way that the results are helpful and the greatest good comes to the plant. In other words the reactions are purposive. These growths are so elaborate and beneficial that they often appear as the result of reason and will. The bending of the root into the soil brings it into contact with water and other foods; the tendril coils about a branch and binds the plant firmly to the support. The plant, however, does not direct these adjustments. These reactions do not involve consciousness. These movements and all others are absolutely directed and controlled by the various forces which act upon the sensitive protoplasm. A definite reaction follows a stimulation which the plant has no power to alter or control. The root will bend down into a dish of mercury with the same energy and directness as that with which it penetrates the soil, and the tendril will clasp your finger or a branch of its own body as firmly as a serviceable support. .

6 AIM OF THE STUDY

We shall now proceed to study the plant from this standpoint, giving attention to the fitness or adaptation of the leaf, root and stem, to the conditions under which the plant lives, and to the work which it performs; and finally we will be interested to learn how the seed is formed and how the plant lives from year ,to year.

PEARY 1 THE NATURE OF PLANTS

CHARTER I

THE LEAF 3. The Work of the Leaf.—Each plant has a character or a personality of its own.. What is it that gives this individuality to the plant? When the work that it performs is understood it will be seen that this character is very largely due to the adaptation of the leaves to the performance of certain duties. The

Fic. 3. Forms of leaves: A, leaf of white birch with netted veins—A, petiole; 6, blade. B, leaf of Solomon’s seal with parallel venation and blade clasping stem without petiole. ; 5 leaves bring the plant into harmony with its surroundings and

give to it a subtle individuality owing to the perfection of their

arrangements, structures and forms for the work in hand. The i

8 THE WORK OF THE LEAF more important work performed by the leaves is the construction ‘of foods, the giving off of water or transpiration, and breathing or respiration. The magnitude of this work far exceeds the energy expended in all the industries of the world. The leaves, however, accomplish this work so quietly and economically that " most people are scarcely conscious of it. .

In order to understand the purpose of the petiole and green blade (Fig. 3) or the significance of the various forms and arrangements of leaves it will be necessary to examine the structure of the leaf and see the character of the apparatus that is used in the performance of its work. The blades of the majority

Fic. 4. Blade of lilac leaf cut across, showing the more compact arrangement of cells upon the upper side of the leaf.

of leaves are flat. A section through such a blade, cut as in Fig. 4 so that we can look into the end of the blade, shows that the leaf is composed of a complicated arrangement of cells. Fig. 5 is a greatly enlarged view of Fig. 4 taken at X. :

4. The Epidermis.—It is now seen that a layer of compact cells (Fig. 5, e) surrounds the leaf on all sides. In fact such a layer covers all parts of the plant body. This layer of cells, the epidermis, is provided with minute openings or stomata (sing. stoma), Fig. 5, s. The stomata are especially abundant in the epidermis on the under surface of the leaf and often quite lacking from the upper surface. A better idea of these openings and of the epidermal cells may be gained by stripping off the epidermis cose e)\®

SBS

Ho

S IEG

—— ANH

Fic. 5. Greatly enlarged view of region X in Fig. 4: e, epidermis; s, stoma; Pp, palisade mesophyll; ch, chloroplast; sp, spongy mesophyll; 7, intercellular spaces; v, small “vein cut across; /, end of vein seen from the side, consisting of elongated and banded cells (tracheids); col, collecting cells that transfer material to and from the palisade cells——H. O. Hanson.

SB eS98q ss

Fic. 6. Surface view of epidermis of lilac leaf: g, guard cell; s, stoma; ch, chloroplasts; ~, nucleus.—H. O. Hanson.

10 STRUCTURE OF THE LEAF - from such leaves as the live-for-ever, blue flag or hyacinth. The cells of the epidermis are very compactly put together and may be stripped off by means of a penknife as a thin white skin. Examined under a microscope, such preparations show the stomata as minute openings guarded by two rather lens-shaped cells, termed the guard cells (Fig. 6). Chloroplasts usually occur only in the guard cells, the other epidermal cells having a watery, colorless cell content. In addition to these features, Fig. 7 shows that the outer portion of the walls of the epidermal cells is modified so as to form a delicate, skin-like layer over the outer sur-

Fic. 7. Section across a stoma shown in Fig. 6: s, stoma; g, guard cell: c, cuticle; a, air chamber.—H. O. Hanson.

Fic. 8. Hairs from the epidermis of squash leaf.

face of the epidermis. This part of the cell wall is called the cuticle. It contains a fatty substance, cutin, that renders the epidermis nearly impervious to gases and fluids, so that the leaf is entirely surrounded by a water- and gas-proof coat. In many leaves hairs of various forms grow out from the epidermal cells (Fig. 8).

5. The Mesophyll.—The cells within the epidermis have, for the most parts, a greenish color due to numerous green bodies, chloroplasts, contained in the cells (Fig. 5, ch). These green cells of the leaf, termed the mesophyll; consist of one or more layers of rather elongated and compact cells on the upper side of the leaf (the palisade mesophyll, Fig. 5, ), and below they i i

NATURE OF PLANTS II are joined to rather irregular and loosely arranged cells (the spongy mesophyll, Fig. 5, sp). Numerous air spaces extend in all directions through this spongy mass of cells reaching up to and between the palisade cells and down to the stomata (Fig. 5,4). Bundles of small cells also appear here and there between the palisade and spongy mesophyll (Fig. 5, v, J). These structures are the so-called veins that appear as fine lines in the majority of leaves (Fig. 3). These veins or vascular bundles branch again and again and so extend to all parts of the leaf. In Fig. 5, » one of the veins has been cut square across while at J the end of a small vein is shown, having been cut in half lengthwise. The walls of the mesophyll cells are smooth, thin; and elastic and composed of a substance called cellulose. Thin

_ places or pores are often formed in the walls which appear as openings owing to the extreme thinness of the wall at such places.

_ The term parenchyma is applied to all cells possessing the character of wallseted in the mesophyll cells. In the vascular bundle, however, a portion of the cells have thicker walls which are more rigid, hard and woody. Such cells are often referred to as prosenchyma.

What is the significance of the peculiar and varied arrangements of the cells in the leaf? We naturally anticipate that these different structures are adapted to performance of special duties. Attention will first be directed to the work performed by the green portion of the leaf, the mesophyll. Similar cells containing chloroplasts are of common occurrence in various parts of the plant, being often found in stems and unripe fruits. In the leaf such a tissue is termed mesophyll but a more general term, Chlorenchyma, includes all chlorophyll-bearing tissue wherever found.

6. The First Function of the Leaf, Photosynthesis.—Chlorenchyma is the most important tissue in the plant since it has for its special function or work the construction of carbohydrates, such as sugars and starches, which are among the most important foods of the plant. The construction of these carbohydrates by the chlorenchyma is called photosynthesis. This work easily ranks as the leading wonder of the world because its processes are so complicated and of such magnitude and importance in the:

12 NATURE OF CARBOHYDRATES economy of this world of ours. It will be seen that this tissue - not only keeps the air in a wholesome condition for breathing and so makes life possible upon the earth byt that all animal life is absolutely dependent, either directly or indirectly, for food upon the products built up by the chlorenchyma. Let us see how these foods are formed. Various substances are absorbed by the roots, and the vascular bundles, which extend throughout the plant body, transfer them to the chlorenchyma. This absorbed material is in large part water, but various mineral ‘substances are dissolved in it. Various gases, as carbon dioxide, also find their way through the stoma to the chlorenchyma. All of these absorbed substances are composed of elements. Thus water consists of two elements, hydrogen and oxygen, in the proportion of two parts,of hydrogen to one part oxygen. This composition is indicated by the abbreviation, H.O.' So the gas, carbon dioxide, contains one part of carbon and two parts of oxygen, indicated thus CO;. Chloroplasts have the power under suitable conditions of decomposing or tearing apart these compounds and of reuniting their elements into new compounds that are quite different from the original ones. In this way CO, and H,O are decomposed and the elements reunited into complex «compounds or foods consisting of carbon, hydrogen and oxygen. ‘Such complex foods as cane sugar, consisting of twelve parts of carbon, twenty-two of hydrogen, and eleven of oxygen are formed im this way. This very common food is expressed by the formula CyH»On. So also other foods are formed, as grape sugar C.H,.0, and starch usually indicated as some multiple of CgHi0Os. The cellulose walls of the parenchyma cells have the same composition as starch. It will be seen that all these foods have carbon united to hydrogen and oxygen in the same proportion as it exists in water. Cane sugar has carbon united to eleven times H,.O, and starch five times H;O. It was supposed that these substances were a combination of carbon and H.O. For this reason they were called carbohydrates. This name is still applied to these foods although it is known that the base is not H,O but the hydroxyl OH. The actual changes that are effected in the formation of these various carbohydrates are not known.

It has been suggested that CO, and H.O enter into solution forming carbonic acid (H.:CO;). This change is represented by the equation, CO, + H.0 = H.CO;. By giving off one part of oxygen this acid is changed to formic acid (CH:O,), 41,2; HCO, = CH,O, + O. Asimilar decomposition is effected in the formic acid which results.in the formation of formaldehyde (CH,0), 7. €., CHO, = CH.0 +0. The two parts of oxygen pass from the cells and escape through the stomata as a gas. In the presence of certain alkalies and acids, formaldehyde increases the number of elements which compose it, thus six times CH,O would give C.Hi,0 or grape sugar. This is but a theory based upon the facts that CO, and H,O do unite in nature to form H;CO; and that this acid may decompose as stated above, and finally that formaldehyde can be detected in plant cells and it is possible to produce sugar from this substance by treating it with alkalies and acids. All that can be definitely stated about the changes going on in the leaf, during the formation of the carbohydrates relates solely to the beginning and end of the process. Water and carbon dioxide enter the chlorenchyma cells. A series of changes follows, the nature of which can only be conjectured. Finally, as the end result, sugar appears in the cells and oxygen is set free, escaping as a gas.

These complex changes are brought about in the leaf in so subtle a way that we are not conscious of them or of the great amount of energy that is required to effect them. It requires the energy expressed by a temperature of 1300° C. to decompose CO; into its elements. Where does the plant obtain the energy to bring about the decompositions and recomposition? This work is accomplished by the energy of the sunlight acting upon the chloroplasts. The chloroplasts of the seed plants are minute, rather lens-shaped grains, and increase in number by the division _of the plastid into two equal parts (Fig. 9). These bodies are denser portions of the cytoplasm and like it are nearly colorless. In the presence of light a green, oily substance, chlorophyll, is formed in the plastids, thus producing their green color. Chlorophyll is rarely formed except in the presence of light. Plants grown in cellars or in the dark are of a pale color owing to the

14 NATURE OF CHLOROPHYLL absence of chlorophyll in the plastids. Similarly green plants lose their chlorophyll when placed in the dark. This is the principle employed in the blanching of celery. The stalks are surrounded by earth or the light is excluded from them by some other means; chlorophyll disappears from the plastids and fails to develop in the new cells that are formed in the dark. Plants in which the chlorophyll is not developed are said to be etiolated. The plastids are in the cells, however, for if an etiolated seedling is removed from the dark to the light the chlorophyll will begin to appear in a few hours. The greening of potatoes when removed from the ground and left in a strong light is a familiar example of this. It is evident that the formation of chlorophyll is usually closely connected with the action of light. Another important point regarding the influence of light is the fact that such a food as starch is not usually found in the leaf in the absence of light. This may be easily verified by cutting the initials of your name in rather large letters in a strip of black paper or smooth tin-foil and fastening the strip in the early morning to the upper surface of a well-sunned leaf of a starch forming plant. After a few hours remove the leaf from the plant and place it in alcohol. This will dissolve the chlorophyll and the leaf will become quite white in a few hours. Now place the leaf in a tincture of iodine which turns starch to a blue or blue-black color.

Wherever the light was excluded from the leaf there is only a pale yellow color but all portions of the sunned leaf show an abundance of starch as revealed by the blue or blue-black coloration. Consequently the letters appear black on a yellow background which marks the limits of the strip of paper. No starch can be detected as a rule in leaves taken from plants that have been growing in the dark for twenty-four hours. Plastids that do not contain chlorophyll, examples of which are found in the cells forming the white bands or blotches in many . variegated plants, do not form starch in the light. It is evident from these facts that the chlorophyll and light co-operate in some way in the formation of starch. It is known that chlorophyll absorbs a certain portion of the sunlight. If a beam of sunlight is caused to pass through a prism it is separated into seven colors, 7. e., red, orange, yellow, green, blue, indigo and violet. When, however, a beam first passes through an alcoholic solution of chlorophyll or through a green leaf a portion of certain colors and all of other colors do not appear. The chlorophyll has taken up or absorbed certain of the light rays especially portions of the red, blue and violet. It is reasonable to suppose that the energy in these rays is utilized in part by the plastids in effecting the changes noted above in the formation of foods. The sun gives to the earth great quantities of energy in the form of light. A portion of this light is taken up by the chlorophyll, and the plastid uses the energy of the light to bring about the recombination of CO, and H,O and the formation of sugars, starches, and other carbohydrates. Because of the importance of the sunlight in the formation of carbohydrates this process is called photosynthesis. This term means the putting together by means of light.

7. The Magnitude of the Work of Photosynthesis.—Carbon is an important element in the composition of plant foods and also in the walls of the cells. It forms one half of the dry weight of the plant. All the carbon appearing in the plant is derived solely from the CO; in the air, of which it forms a very small part, only about three parts in ten thousand. Furthermore the carbon comprises only 3/11 by weight of the CO:, 8/11 being oxygen. A square meter of leaf surface, however, can withdraw all the CO, from 2500 liters of air in one hour. Such a volume of air would be a space one meter square and 2!4 meters high. This would furnish sufficient carbon for the construction of one gram of starch. In this way such large quantities of CO, are drawn into the chlorenchyma cells as to make possible each year the harvest and the renewal of vegetation of the earth. In the United States alone in this way there was in 1916 built up over 4600 millions of pounds of cotton, 839 million bushels of wheat, 1251 million bushels of oats, etc., representing a value for all crops of 8 billion dollars. Our leading crop is corn which amounted to over 21% billion bushels and valued at about 214 billion dollars. The gold and silver coin and bullion of the country are not of greater value. This corn came up and matured in 120

16 NATURE OF PROTEIDS days, consequently these coin plants were manufacturing food at the rate of over 19 million a day. These figures represent only a small portion of the work performed, since the grain is but a part of the plant, and furthermore the larger portion of the land is covered with forests and other forms of vegetation. The fact must not be overlooked that this process of photosynthesis is of vital importance to our welfare in another way. Owing to the large volumes of CO, that are constantly formed by fires and the -respiration of animals such an excess of this gas would accumulate in the atmosphere that all animal life would eventually cease were it not for its absorption in photosynthesis. So balanced are the rates of formation and absorption, however, that the percentage existing in the air does not materially vary.

8. The Construction of Proteids.—A second group of foods formed by plants are called proteids. These differ from the carbohydrates in that they contain nitrogen in addition to carbon, hydrogen and oxygen. They are more complex compounds than the carbohydrates and may contain sulphur and phosphorus in addition to the four elements mentioned above. Less is known about their formation than of the carbohydrates. It is probable that they are largely formed in the leaves and by a process similar to that of photosynthesis. The sulphates, phosphates, and nitrates absorbed from the soil are decomposed and the elements of nitrogen, sulphur or phosphorus are united to simple carbon compounds and complex proteids are the result. Light probably does not co-operate directly in this construction although it may do so indirecily. These foods are formed in much smaller quantities than the carbohydrates but they are of the greatest importance in the nourishment of the plant. This is especially true as regards the living substance, protoplasm, which resembles somewhat in composition some of the more complex proteids.

9. The Distribution of the Foods.—Let us now consider what becomes of these foods. A small part is consumed on the spot by the manufacturing cells themselves, a larger portion is transported through the vascular bundles to all the living and growing cells of the plant body, but as the plant approaches the completion of its annual growth a larger and larger part of the food is transferred to special parts of the plant, such as buds, roots, seeds. Here it is stored as a reserve food to meet the needs of the plant at such times as it is not able to manufacture food. This transfer of foods is slow and consequently the rapidly constructed sugars gradually accumulate in the chlorenchyma during the day. This would result in the saturation of the cells with sugar and so stop the work of photosynthesis were it not for the fact that the chloroplasts quickly change the sugar to insoluble starch, thus leaving the cells free to receive more sugar. If chloroplasts of well-sunned, starch-forming leaves are examined in the afternoon, they will be found to contain minute glistening bodies, the starch grains (Fig. 9, s). The chloroplasts have the power to absorb sugar and secrete starch. The construction of