THE PLANT: ITS STRUCTURE, LIFE -PROCESSES AND ENVIRONMENT 



15 



contained in the water. The leaf thus becomes the 

 meeting place of air food and soil food. These two 

 sorts of crude food combine to form elaborated 

 food. The first step is probably the formation 

 cf sugar, which then, by combining with nitrogen, 

 sulfur, phosphorus and other elements, forms pro- 

 teids. .These move from place to place, principally 

 in the bast, and so reach the regions where they 

 are needed. 



The energy needed to elaborate food comes from 

 the sunlight. The leaves have various devices to 

 absorb all the sunlight possible. Some " follow the 

 sun " all day. long, thus facing eastward in the 

 morning and westward at evening. At mid-day they 

 are horizontal, except when the sunlight is exces- 

 sive, in which case they assume the " profile posi- 

 tion" with the edges pointing upward, thus avoid- 

 ing injury due to too strong light. Many such 

 leaves assume a " sleep position " at night by fold- 

 ing ; they diminish thereby the loss of heat and 

 avoid the precipitation of dew on the protected 

 surfaces. 



Most leaves have the power of turning toward 

 the light, and so move out of the shadow of other 

 leaves. Thus arise the beautiful " leaf -mosaics," 

 e. g., of English ivy or of maple, in which no leaf 

 unduly shades another. The usual arrangement of 

 leaves on the stem is in regular vertical rows. 

 The arrangement is known as phyllotaxy. 



The stem. — The stem bears the leaves and fur- 

 nishes them with a constant supply of water, which 

 it conveys from the root. On placing a plant with 

 its roots in diluted red ink or other colored solution, 

 we can trace the colored solution up through the 

 wood-cells in the root (Pig. 29), through the stem 

 (Fig. 36), into the finest veins of the leaf. It is 

 easily seen that the colored solution travels only in 

 the wood-cells and not in the other cells of the stem. 

 We usually find the wood-cells associated with 

 bast-cells, forming together the fibre- vascular bun- 

 dles (Figs. 21, 37). In dicotyledonous plants (e. g., 

 squash, sunflower) these bundles form a circle near 

 the outside of the stem ; while in monocotyledonous 

 plants (e. g., corn, lilies) they are scattered through 

 the stem. [See page 9.] 



The largest passages in the wood are called 

 ducts, and in them the water travels faster tL..n 



Fig 35 A chlorophyll 

 gram containing young 

 starch grains. The 

 dotted shading is to in- 

 dicate the chlorophyll 

 drops. 



Fig. 36. Diagram of 

 cross-section of 

 squash stem. Btr, 

 strengthening fibers. 



in the other cells. They are formed by the breaking 

 down of partitions, thus converting a long row of 

 cells into a single continuous passage that may be 

 as much as forty feet in length. 



In the tracheids — long, narrow, tapering cells — ' 

 the water travels more slowly than in the ducts, 

 being hindered by the frequent end walls. The 



markings seen on the walls of the wood-cells are 

 pits or thin places in the walls, by means of which 

 water passes more readily from one cell to another. 

 The passage of air is prevented by a delicate mem- 

 brane stretched across the pit. 



The question may be asked, What causes the 

 sap to rise ? Various explanations have been 



Fig. 37. Fibro-vascular bundle (cross-section) of Indian corn, 

 much magnified, a, annular vessel; a', annular or spiral 

 vessel; t, thiek-walled vessels: w, tracheids or ■woody 

 tissue; f, sheath of fibrous tissue surrounding the biindle; 

 ft, fundamental tissxie or pith; s, sieve tissue; p, sieve 

 plate; c, companion cell; i, intercellular space, formed by 

 tearing down of adjacent cells; w', wood parenchyma. 



advanced and proved unsatisfactory, such as capil- 

 larity, barometric pressure, action of air-bubbles 

 and root-pressure (the action of the root in forcing 

 water upward, as seen in the bleeding stumps of 

 the grape-vine). The one at present most in favor 

 is that the sap is drawn up by water-attracting 

 substances in the leaves, just as the water is pulled 

 away from the soil -particles by the root -hairs. 

 This process is known as osmosis. Sugar is a sub- 

 stance that acts in this way. For example, the 

 conversion of the stored starch of the maple into 

 sugar, in the spring, causes a rapid rush of sap 

 into the stem, even though no leaves are present. 

 This theory is not .satisfactory in all respects, 

 especially when applied to the rise of sap in very 

 tall trees. 



Among the wood-cells are found short cells, 

 wood-parenchyma, that remain alive long after the 

 other cells are dead. One of their chief functions 

 is to store starch and other foods that are conveyed 

 to them by the medullary rays or silver grain. These 

 consist of elongated cells that run at right angles to 

 the course of the wood-cells ; they serve to convey 

 gases as well as food. Much elaborated food, espe- 

 cially proteids, is conveyed by the bast. Most pro- 

 teids are unable to pass through cell-walls and so 

 are able to move only in the large cells, or sieve- 

 tubes of the bast, whose end walls or sieve plates 

 are pierced with holes. The bast contains smaller 

 cells known as companion cells and bast parenchyma 



