14 



THE PLANT: ITS STRUCTURE, LIFE - PROCESSES AND ENVIRONMENT 



The stomata are usually closed at night; hence it is 

 then possible to fumigate plants with poisonous 

 gases that would kill them if applied through the 

 day. Closing at night prevents the stomata clog- 



Pig. 32. Diagram of a stoma or stomate (of Iris) in section, 

 ing guard-ceUs and neighboring ceUs of epidermis. 



ging with dew. Water-proof materials, as well as 

 hairy coverings of the leaf, protect the stomata 

 from dew and rain. Leaves so protected appear 

 silvery under water and do not become wet for a 

 long time. If such protection is found on the lower 

 side only; the stomata will be found on that side 

 only. House plants should have the leaves washed 

 occasionally to prevent the clogging of the stomata 

 with dust. The devices by which desert plants 

 check evaporation will be discussed later. 



The carbon dioxid, passing through the stomata, 

 comes directly into contact with the leaf-cells, 

 which are sufficiently separated from each other to 

 allow it to pass freely between them (Pigs. 33, 34). 

 The great absorptive surface which they expose is 

 kept continually moist and is thus able to absorb 

 with great rapidity, much as the moist lung sur- 

 faces absorb oxygen. The absorbed carbon dioxid 

 passes into the cells and comes into contact with 

 the green chlorophyll grains. The chlorophyll (leaf- 

 green) in these bodies is divided into very minute 

 drops (Fig. 35), thus giving it an enormous ab- 

 sorptive surface. At the same time that it takes 

 up carbon dioxid it absorbs sunlight, and with the 

 energy thus received decomposes the carbon dioxid 



Fig. 33. Cross-section of Ivy leaf, which grew in shade, and 

 has only one layer of paUsade-cells. u, upper epidermis; 

 p, palisade cells; c, a crystal; sp, spongy parenchyma; 

 i, intercellular space; I, lower epidermis. The plant here 

 intended is the true or English ivy, Hedera helix. 



and causes the carbon to unite with the water, 

 thus forming sugar. This may be illustrated by the 

 equation : 



6CO2 + 6H2O = CeHiaOo + 6O2 

 Carbon dioxid Watei Qrape sugar Oxygen 



This equation, however, states merely the begin- 

 ning and end of what is probably a long and com- 

 plicated process. Oxygen is given off and may be 

 seen arising in bubbles from water plants. Air 

 that has been "vitiated" by animals may 

 have its oxygen restored by green plants in 

 sunlight. Aquaria are often maintained for 

 long periods when a proper balance is struck 

 between plant and animal life. 



The process just described (photosynthe- 

 sis) furnishes not only all the food, but prac- 

 tically all the fuel in the world. The leaf 

 utilizes, that is, stores up, only about one- 

 half of one per cent of the energy it re- 

 ceives in the form of sunshine. It makes 

 use of the red and orange rays almost exclu- 

 sively, and forms little or no starch in blue 

 light. The rays that affect the photographic 

 plate, therefore, have little part in photo- 

 synthesis, while the red and orange rays, so 

 important in this connection, are the ones 

 that also produce the greatest effect on the 

 eye. A square meter of sunflower leaves 

 is estimated to produce about 25 grams of starch 

 in the 15 hours of sunlight of a summer day. This 

 would use up the carbon dioxid contained in 50 

 cubic meters of air (a meter is nearly 40 inches); 

 or, in other words, should the leaves take all their 



show- 



Fig. 34. Leaf of common wild yellow mustard; e, epidermis; 

 p, palisade cells ; 8P, spongy parenchyma; col, collecting 

 cells; conv, conveying cells; sh, conducting sheath of vein; 

 w, woody tissue of vein; 6, bast of vein; s, stoma; a, air- 

 space; chl. gr., chlorophyll granules. 



carbon from the air directly above them, they 

 would in a day consume all of it to a vertical 

 height of about 165 feet. 



The sugar formed in the chlorophyll grains is 

 transformed, in great part at least, into starch, 

 which makes its appearance in the form of glis- 

 tening white bodies embedded within the substance 

 of the grain. This starch mostly disappears during 

 the night, being changed back into sugar, and 

 conducted away into the stem and thence to the 

 roots, flowers or other parts. Leaving the palisade 

 cells of the leaf (Fig. 34 p.), it passes through the 

 collecting cells (col.) into conveying cells (conv.), 

 and on to the conducting sheath (sh.) of one of the 

 veins, by which it passes through the leaf-stalk 

 into the stem. 



The evaporation of water is of great advantage 

 to the plant, for it concentrates in the leaf the salts 



