254 - Multicellular Plants 



without which the synthetic and respon- 

 sive activities of the plant could not pro- 

 ceed. 



The mainspring of constructive metabo- 

 lism is photosynthesis, which is effected 

 chiefly in the leaves. Light energy provides 

 for the synthesis of various organic mole- 

 cules (p. 164), among which glucose is very 

 important. This sugar (or its derivatives) 

 provides a source of matter and energy in 

 the cells throughout the whole plant. 



Under average conditions the amount of 

 glucose produced in a leaf is about 1 gram 

 per square meter of surface per hour of day- 

 light. In the present period of geological 

 time one limiting factor imposed upon the 

 rate of photosynthesis is the relatively small 

 concentration (about 0.03 percent) of C0 2 

 in the atmosphere; if a greater amount (up 

 to 10 percent) of C0 2 be provided artificially 

 (as is done in some modern greenhouses) the 

 rate of photosynthesis can be accelerated 

 considerably. This, in fact, is part of the evi- 

 dence indicating that the tremendous growth 

 of vegetation that occurred during the era 

 when our coal deposits were formed resulted 

 from a higher content of COo in the terres- 

 trial atmosphere. 



In daylight, the production of glucose ex- 

 ceeds the rate at which sugar can be evacu- 

 ated from the leaf, and the excess is con- 

 verted into starch in the chlorenchyma cells. 

 At sunset, accordingly, the leaf contains an 

 abundance of starch — as can be demonstrated 

 by the iodine test. But at sunrise the test is 

 apt to be negative. During the night the 

 temporary deposits of starch are reconverted 

 to glucose, which gradually passes from the 

 leaf to other parts of the plant. 



Aside from photosynthesis, constructive 

 metabolism is essentially similar in the differ- 

 ent tissues of the plant. Given glucose, to- 

 gether with nitrates, sulfates, phosphates, and 

 other inorganic salts, plant cells can syn- 

 thesize all the different amino acids; and 

 from the amino acids, by dehydration syn- 

 thesis, each cell forms the specific proteins 

 essential to its protoplasmic structure. Al- 



though all the cells of a plant can synthesize 

 amino acids, the chlorenchyma cells — having 

 a direct supply of light energy — are in a 

 specially favorable position. Accordingly the 

 green tissues of the plant tend to produce 

 extra amino acids and to transmit the extra 

 amounts to the lower parts of the plant via 

 the sieve tubes. 



Plant cells also can convert glucose into 

 fats and steroids; and if phosphates and other 

 salts are available, glucose can be converted 

 into phospholipids. Thus it may be said that 

 plants use glucose as a source of carbon, 

 hydrogen, and oxygen in synthesizing all 

 their organic requirements, including such 

 other compounds as vitamins, special drugs, 

 perfumes, etc. 



Destructive metabolism in plants is not 

 quite as complex as in animals, because 

 plants derive most of their energy from the 

 catabolism of glucose. Except in seeds and 

 certain other regions where reserve supplies 

 are necessary, plants tend to limit their syn- 

 thesis of proteins, lipids, and other structural 

 components of the protoplasm to such 

 amounts as are needed for growth and main- 

 tenance. Consequently plants do not usually 

 possess excesses of these structural com- 

 pounds for use as fuel. During periods of 

 malnutrition, especially when light is lack- 

 ing or inadequate, plants may sacrifice some 

 of their proteins and other essential com- 

 pounds in order to obtain energy for main- 

 taining the protoplasmic structure — but this 

 last resort postpones death only for a short 

 while. Ordinarily plants derive virtually all 

 energy from carbohydrates, and consequently 

 the end products of plant catabolism are 

 mainly C0 2 and FLO. 



Because of photosynthesis, the constructive 

 metabolism of plants greatly exceeds destruc- 

 tive metabolism. In growing, an average 

 plant oxidizes only about 15 percent of its 

 photosynthesized glucose. This oxidized glu- 

 cose liberates enough energy to convert the 

 remaining glucose into starch (about 50 per- 

 cent); into cellulose and related compounds 

 (about 25 percent); and into the carbon- 



