102 TEXTBOOK OF BOTANY 



nitrogen, carbon, and their simplest compounds were recognized and 

 their nature was understood. Another century had nearly passed before 

 the food of green plants was discovered. 



Aristotle was further impressed by the absence of excretory organs in 

 plants, but concluded that the soil in some way acted as the stomach of 

 the plant and supplied it with only perfect food, from which no unused 

 products would accumulate and have to be eliminated. For centuries 

 these two notions of Aristotle prevailed and even now they have not 

 entirely disappeared. 



The food of green plants not in the soil. During the latter half of the 

 last century students of plant physiology not only discarded the idea 

 that green plants get their food from the soil, but they discovered how 

 and where this food is made. In spite of all these well-publicized discov- 

 eries, the mistaken ideas of Aristotle are still current in conversation, 

 advertising propaganda, public addresses, and modern literature three- 

 fourths of a century after they were adequately disproved. 



Even before the numerous discoveries of the 19th century, van Hel- 

 mont (1577-1644), a Flemish physician and chemist, showed by a sim- 

 ple experiment that Aristotle's ideas about the food of green plants were 

 erroneous. Van Helmont placed a willow branch in a tub of soil to 

 which thereafter only rain water was added for a period of five years. 

 At the conclusion of the experiment the willow branch had become a 

 small tree and had gained 2627 ounces in weight, not including the 

 leaves that had fallen each year. The weight of the soil in the tub de- 

 creased but two ounces. 



In both animals and plants it is the protoplasm for which food is 

 necessary. We have already seen that the protoplasm of all organisms 

 is similar in its gross chemical composition. From these two facts alone 

 it appears that the food of green plants must be similar to that of ani- 

 mals regardless of how it is obtained. 



A part of man's food is derived from seeds, bulbs, tubers, and roots. 

 If these same plant organs are placed in a vessel containing only pure 

 quartz sand and a little water, small plants presently grow from them. 

 The same results are often attained by merely placing the seeds in moist 

 air. Small plants will grow from detached bryophyllum leaves suspended 

 near a window. 



During the growth of these young plants millions of new cells are 

 made, each composed of protoplasm and cell wall substances. The proc- 

 esses involved in forming new masses of cells require chemical energy 

 in addition to foods. One cannot avoid the conclusion that seeds, bulbs. 



