Juvenile vs Adult Leaves

seedling of arbor vitae are needle-shaped (Fig. 91) ; but after one or more seasons' growth, scale-like appressed leaves develop and ordinarily continue to be formed during the life of the tree. Another ex- A B FIG. 90. Aerial (A) and submerged (B) leaves of a water crowfoot. Modified from Goebel. ample of "juvenile" leaves is seen in the bean, whose first- formed foliage leaves are undivided whereas those formed later are compound. LEAVES 109 Leaves of juvenile form are not confined to seedlings; not uncommonly, leaves borne on branches developed from adventitious A buds formed in a callus are of the juvenile type. change in the environment of an older plant, too, may result in the formation of juvenile leaves. The basal (juvenile) leaves of the harebell are rounded, whereas the leaves borne on the upper portion of the stem are long and slender (Fig. 92). An environmental change, such as a marked change in il- lumination, causes a stoppage of terminal growth, followed by the development of lateral shoots whose basal leaves are rounded. 66. Fall of Foliage Leaves. Intemperate climates the autumnal shedding of leaves by dicotyledonous trees and shrubs is a well- known phenomenon. It is brought about by the development of FIG. 91. Seedlings of the arbor vitae bearing leaves of juvenile and adult forms. a special layer of cells (an abscission layer. Fig. 93) across the base of each petiole, and sometimes, in a compound leaf, across the base of the stalk of each separate leaflet as well The cell walls of the abscission layer are thin; the middle layer of each wall becomes dissolved, and finally nearly the whole thickness of the wall is softened and dissolved. The abscission layer does not extend across the vessels and tracheids of the bundle or bundles, whose walls, however, are easily broken by the wind or by the weight of the leaf after the disintegration of the walls of other cells. In some 110 GENERAL BOTANY oaks and other trees, the abscission layer is not well developed in the autumn; dead leaves, therefore, may remain on such trees well into the winter or even into the spring. The cells of the basal part of a petiole immediately below the abscission layer usually develop into a corky tissue which is externally visible on the stem as a leaf scar. The fall of a scale leaf, like that of a foliage leaf, is brought about through the formation of an ab- scission layer. Many trees and shrubs indigenous to regions without pronounced seasonal changes do not shed all their leaves sirnul- Vascular Bundle Petiole Cork FIG. 92. Harebell (Campanula rotundifolia), with leaves of juvenile form at the base, and of adult form on the upright stalk. \ Xylem Phloem FIG. 93. Diagram showing the attachment of a leaf petiole to the stem, and the position of the abscission layer. taneously. These plants, exemplified by eucalyptus, oranges, and live oaks, form new leaves and shed old ones continuously through- out the entire year. Such " evergreen " plants are always in foliage. LEAVES 111 67. Scale Leaves. In many cases a leaf primordium matures into a flattened leaf which is attached to the stem by a broad base, and which carries on little or no food-manufacture. Such a scale leaf is usually relatively small, without chloroplasts, and brownish or yellowish in color. Scale leaves about a protected bud prevent mechanical injury of the embryonic parts within. They aid also in checking evaporation from structures within the bud, and so minimize the harmful effects of sudden changes in temperature. The scale leaves of some buds are coated with resin, as in the poplars, and they may be provided with a dense coating of hairs, as are the inner scale leaves of a horse- chestnut bud. Frequently there is no sharp distinction between scale and foliage leaves, and often, as in the lilac, there are all gradations from scale leaves at the outside of a bud to foliage leaves within. After foliage leaves have emerged from the bud, each scale leaf usually falls away. In some buds, like those of the hickory, the inner scale leaves become large and brightly colored before they fall. Scale leaves develop on subterranean, as well as on aerial, stems and branches. FIG . 94. Leaves of the The scale leaves surrounding the embryonic region of a subterranean stem or branch constitute a protective sheath nasturtium (Tropaeolum), whose petioles function as tendrils. which prevents the abrasion of the embryonic region as the stem pushes through the soil. 68. Tendrils and Spines. An entire leaf primordium or a portion A only of such a primordium may mature into a tendril. tendril, therefore, may represent a whole leaf or only a part of a leaf. In peas (Fig. 76, A) and vetches one or more leaflets toward the terminal end of the leaf are tendrils. In some smilaxes (not in- cluding the greenhouse "smilax," which is an asparagus), the stipules are tendrils. In clematis and the nasturtium (Fig. 94) the petioles may function as tendrils, winding about a support and enabling the plant to climb. The term " tendril" is, in fact, applied to any twining portion of a plant which helps to attach the plant to a supporting object. The twining organs of the 112 GENERAL BOTANY grape (Fig. 58) are tendrils, although they are branches rather than leaves. Spines and thorns, likewise, may be branches, leaves, parts of leaves, or in some cases roots. The common barberry has one to five (typically three) spines at each node (Fig. 95), the spine or group of spines in each case representing a leaf. Some of the spines of cacti (Fig. 96) are leaves and some are branches. SUMMARY Leaves differ from other vegetative organs in that in general all their cells enlarge and mature simulta- neously. Mature leaves may be op- posite, whorled, or alternate in ar- rangement on the stem. A foliage FIG. Winter condition of a leaf is commonly stem of the com- composed of mon barberry; the spines are leaves of special form. blade, petiole, and stipuleg' but gome , lack petiole, stip- ules, or both. Leaf blades are either parallel-veined or netted-veined. In the latter case the arrangement of veins may be palmate or pinnate. When marginal lobes of a blade ex- tend to the midrib or to the base of the blade, the leaf is. compound. Leaflets of a compound leaf may be FIG. 96. Portion of the stem of pinnately or palmately arranged. Leaflets may themselves be divided. Apart from veins, the blade of a a cactus (Carnegiea gigantea), bearing spines and flowers. Photograph by D. T. Mac- dougal. dicotyledonous leaf usually consists of the following tissues: upper epidermis, palisade tissue, spongy tissue, lower epidermis. Both lower and upper epidermis may A contain stomata. stoma is a space between two guard cells, Veins are vascular bundles, continuous with those of the steiE; LEAVES 113 and consisting of the same primary tissues. Surrounding a vein is a sheath composed of parenchyma and including, in the cases A of many large veins, mechanical tissue as well. petiole contains one or more vascular bundles which connect the vascular bundles of the stem with the veins of the leaf blade. Structural differences between leaves of various plants are due largely to the proportional amounts present of the various tissues already mentioned. Leaves other than foliage leaves may be scale leaves, tendrils, and spines. The separation of leaf from stem is due to an abscission layer formed at the base of the petiole. CHAPTER VIII RELATIONS OF PLANTS TO WATER 69. Importance of Water to Plants. In previous chapters it has been noted that water, together with nutrient substances in solution, is absorbed by roots and is conducted through the vascular tissues of roots, stems, branches, and leaves to all parts of a plant. An ample supply of water is necessary for all the activities of the plant. The importance of water results in part from the fact that it is the liquid in which all or almost all other substances that are to be utilized or are to be moved from cell to cell must be dissolved; and in part from the fact that water is the largest single constituent of protoplasm, which in active cells is ordinarily in a semi-fluid condition. The different organs of a protoplast such as dense cytoplasm, plastids, and nucleus differ in the amount of water they contain, but all parts of a living cell must be nearly saturated with water in order to carry on their ordinary functions. The need of a water supply is greatly increased by the fact that water is constantly being lost by evaporation from cells of all aerial parts of a plant, especially from the leaves; and this loss can be compensated only by an intake of water from the soil or from other available source. As between various organs and tissues, the amount of water present may fluctuate within rather wide limits. In root tips, fruits, and young leaves, the proportion of water may be as high as 90 to 95 per cent. In woody stems the proportion of water is often about 50 per cent; in dormant winter buds 40 to 50 per cent; and in dormant seeds the proportion may be as low as 10 to 15 per cent. The greatest proportion of water is required for activities connected with constructive processes such as those of growth, and the smallest proportion is required for destructive processes such as respiration. Water, however, is necessary to all the physical and chemical changes that occur within the plant. 70. Transpiration. Water evaporates from a free water surface or from a surface containing water; that is, it changes from a liquid state to a vapor and passes into the atmosphere. The evap- 114 RELATIONS OF PLANTS TO WATER 115 oration of water from the exposed surfaces of a plant is transpi- ration. Since in most familiar plants transpiration is chiefly from the cells of leaves, it is in leaves that this process is most readily studied. On the other hand, the structure of a leaf is best under- stood if the leaf is considered in its relation to transpiration. The spongy tissue of a leaf is constantly evaporating water into the intercellular spaces, from which the water vapor passes into the outer atmosphere, mainly through the stomata. The epidermis of a leaf allows some water to pass through it, but the amount of water lost to the atmosphere from the epidermis is relatively small in land plants because of the presence in and on their walls of cutin. That water is lost from the surface of a leaf, and that this loss is mainly through the stomata, may be shown by the following experiment. A geranium leaf is removed from the plant, its lower surface (in which most of the stomata are located) is coated with a layer of wax or vaseline, and the leaf is then laid on a table. An- other similar leaf cut at the same time, whose upper surface only is coated, is placed beside the first. The second leaf will wilt much more quickly than the first. The wilting is caused by the loss of turgidity of the cells of the leaf; the loss of turgidity results from a loss of water. The slower wilting of the leaf whose lower surface was coated is due to the fact that most of the water loss was through the stomata. In various cases, from 80 to 97 per cent of the water lost by transpiration passes through stomata, the remainder being lost through the cutinized epidermis. 71. Amount and Rate of Transpkation. Some conception of the extent to which transpiration goes on may be gained by comparing the loss of weight from a pot containing a geranium plant in soil with the loss from a pot of the same size -containing soil but no plant. The soil in both pots is well watered at the beginning of the experiment, and both pots are wreighed. They are weighed again at the end of 24 hours. The pot containing no plant will be found to have lost some weight because of the evaporation of water from the soil. The other pot will have lost about the same amount in the same way; but the total loss of weight from the pot containing the plant will be much greater. The difference between the losses of weight in the two cases is an approximate measure of the loss1- by transpiration from the leaves of the plant. The approximate rate of transpiration may be determined by A the use of a potometer (Fig. 97). cut shoot is fitted into an up- 116 GENERAL BOTANY right tube (A); to this tube an empty horizontal tube (B) is at- tached whose free end is bent and immersed in water (C). By A opening the stop-cock of the vessel D, water is driven into tubes and B. By removing the end of tube B from the water in C and allowing the plant to transpire for a short time a bubble of air (E) is introduced; the lower end of the tube is then replaced in C. Now, as water evaporates from the plant it is replaced by water drawn FIG. 97. Potometer, an apparatus used to determine the amount and rate of water-absorption and water-loss by a plant during transpiration. E from tube B; the result is a shifting of the bubble toward the plant. By noting the time required for the bubble to move a certain distance, the rate of transpiration may be estimated. If the apparatus is placed under different external conditions, the effects of the environment on transpiration may be studied. The method just described is not a strictly accurate one for determining the rate of transpiration because it measures the rate at which water is being taken into the plant, and the amount of water transpired may be different from the amount absorbed. The RELATIONS OF PLANTS TO WATER 117 amount of transpiration and its relation to the amount of water absorbed can be determined by standing the apparatus just described on one pan of a balance, the other pan being weighted to bring the two to the same level. After a half hour or more the pans will no longer be in balance because of the water lost by transpiration. Such a procedure gives some idea both of the amount of water absorbed by the plant and of the amount lost by tran- spiration. By far the greater part of the water absorbed by plants is lost by transpiration, and the amount lost is surprisingly large. Under ordinary growing conditions, a square foot of the leaf surface of a sunflower transpires about four ounces of water in the course of 24 hours. In a growing season of 100 days, this would imply a A loss of 25 pounds of water per square foot of leaf surface. single corn plant growing in Kansas has been shown to remove 54 gal- y lons or 1 barrels of water from the soil in a single season, which is 90 times as much water as is needed by the plant for all purposes except to replace the loss by transpiration. An apple tree 30 years old may lose 250 pounds of water in a day, or 36,000 pounds during the growing season. At this rate an acre of 40 apple trees would transpire 600 tons of water per year. Experiments show that the amount of water transpired by a plant fluctuates from hour to hour, from day to day, and from season to season. Such fluctuations are due largely to variations in the external conditions, although conditions within the plant and within its individual cells also affect the rate of transpiration. Important among external factors that influence transpiration are the temperature and the humidity of the surrounding air. Winds and air currents affect the process as they affect humidity and temperature. Other things being equal, the drier and warmer the air the more rapid is transpiration. In very moist and cool air transpiration is comparatively slow. Quite apart from the direct effects of the sun's rays upon the temperature of the air, the inten- sity and quality of light are also important in affecting transpira- tion. Green leaves and other green parts of a plant absorb a con- A siderable portion of the light rays falling upon them. small part of the solar energy thus absorbed is used, as will be seen (Chap. IX), in food-manufacture; but in bright light the greater portion is changed to heat and increases the rate of transpiration. Some of this excess heat vaporizes the water which is then lost 118 GENERAL BOTANY in transpiration. The leaf or other transpiring organ is thus cooled, so that its temperature is kept at or near that of the surrounding air. Transpiration, therefore, tends to regulate the temperature and to prevent excessive heating of the organs of the plant, al- though this is not the principal significance of transpiration. As will appear later, transpiration is important also as a factor in the transfer to the aerial portions of the plant of water and mineral salts absorbed by the roots. 72. Functions of Stomata and of Intercellular Spaces. The im- portance of a large leaf surface lies in the fact that it is necessary in the case of a green plant both that a considerable surface be exposed to sunlight, and that a large proportion of its cells have access to certain gases of the air (carbon dioxide and oxygen) which are used by the plant. The large leaves borne by many common plants are adapted to meet these needs ; but the presence of such leaves increases the danger of too rapid transpiration. The aerating system consisting of stomata and intercellular spaces permits, as shown in 62, the exchange of gases by dif- fusion between the cells inside and the air outside the leaf. In order that gases may be absorbed by the plant they must be in solution in water. The gases in the intercellular spaces of a leaf pass into solution in the water in the cell walls; the dissolved gases may then diffuse by osmosis into the protoplasts. At the same time, water evaporates from the surfaces of the walls abutting upon the intercellular spaces. The air in these spaces thus tends to become saturated with water vapor. This water vapor can pass to the outside of the leaf only by diffusion through stomata (Fig. 98). The further movement of the water vapor from the outside sur- face of the leaf depends, among other things, upon the carrying away of the water-laden air by winds and other air movements. Except for the stomata, the leaf is covered by a continuous layer of epidermal cells whose outer walls are in general cutinized and so are relatively impervious to water. Because of the saturation of the air in the intercellular spaces, loss of water from the cells lining these spaces is less rapid than it would be from cells exposing an equal area on the surface of the leaf. The whole arrangement of stomata and intercellular spaces results, therefore, in greatly increasing the area of the leaf that can take in gases from, and give off gases to, the air. The location of the stomata, or of most of them, in the lower surfaces of most leaves results in the loss of RELATIONS OF PLANTS TO WATER 119 less water than would be lost from the same number of stomata in the upper surfaces, which are more or less exposed to the direct rays of the sun. Stomatal transpiration consists, therefore, first, in the evapo- ration of water from the saturated walls of cells lining the intercellular spaces, and second, in the diffusion of the water vapor Upper Epidermis Palisade Tissue Xylem Spongy Tissue Stoma FIG. 98. Diagram showing the paths of movement of water through and out of a leaf. The movement of water in liquid form is indicated by black arrows; that of water in the form of vapor by light arrows. through stomata. The evaporation of water from the walls of cells adjoining an intercellular space tends to dry out the walls. When this drying begins, the walls of each cell imbibe more water from the included protoplast. Withdrawal of water from the protoplast increases the concentration of substances dissolved in the cell sap, and the cell then tends to draw water by osmosis from neighboring cells that contain proportionally more water. The latter cells in like manner draw from their neighbors, and eventually water is withdrawn from the tracheids and vessels of the veins. These elements of the veins are connected by the tracheids and vessels in the xylem of petiole, stem, and root with the cortex of the root. Thus a continuous stream of water is made possible through root, stem, and branches to the leaf, com- 120 GENERAL BOTANY pensating for the loss of water in the form of vapor from the stomata. The great number of stomata in the epidermis strongly favors diffusion of water vapor outward and of gases inward. Although stomata occupy but one to two per cent of the surface area of a leaf, diffusion through them can go on almost as readily as though the interior cells were exposed directly to the outside air. The rate of diffusion through an epidermis containing numerous stomata rarely rises to its possible maximum. 73. Guard Cells. One striking characteristic connected with the functioning of stomata in many plants is the ability of the guard cells to undergo changes in turgidity and so to change the sizes of the stomatal openings. The mechanism controlling the movements of guard cells is rather complex, and the re- sponses of guard cells to en- FIG. 99. A~ stoma in cr^s section, showing its opening and closing in consequence of changes in the turgid- ity of the guard cells. The thick walls of the guard cells in the open (turgid) condition are indicated by diagonal shading; in the closed (non-turgid) condition, by stippling. Adapted from Schwendener. vironmental conditions are also complex. In general, when &guard cejls are turgid the~y are . . arched . and , tA he i. ncl, ud. ed, stoma is wide open, but when they are not turgid they straighten , , , , . . . anc* c * ose or reduce the size of the stoma (Fig. 99). The tur- gidity or non-turgidity of guard cells is, however, affected by a number of factors, chief among which are the intensity of illumination and the water content of the leaf, especially of its guard cells. In most cases, stomata are open in light and closed in darkness. When water is abundant in the leaves the stomata are usually open; they are usually closed when water is deficient. The behavior of guard cells varies in different kinds of plants. In some common plants, such as the potato, cabbage, and beet, stomata are usually open both day and night if the water supply is abundant. On the other hand, in cereals, such as wheat and oats, stomata are always closed at night, and may even close in the daytime if there is a slight deficiency in the water content of the plant. In very many plants the behavior of the stomatal ap- paratus is intermediate between the extremes just mentioned. In leaves with stomata on both sides, those in the upper surface RELATIONS OF PLANTS TO WATER 121 open later and for a shorter time than those in the lower surface. Stomata near the tip open later and close earlier than those near the base of a leaf. So many factors affect transpiration, however, that this process is not always most rapid when the stomata are most widely open; and