120 CELLS AND TISSUES 



cloth. Consequently, osmotic pressure is lost when cells die and 

 the substances ordinarily retained are allowed to diffuse out. 

 This is easily demonstrated by soaking plant tissues in water be- 

 fore and after death. Thus, if from a fresh red Beet a strip is cut, 

 washed thoroughly so as to remove the contents of the injured 

 cells, and then soaked in water at a temperature not destructive 

 to the life of the cell, it will be found that the pigment, sugar, and 

 other substances of the cell are retained; but if the strips are put 

 in water hot enough to kill the cells, then the pigment, sugar, and 

 other cell substances diffuse out into the water. That pools in 

 which dead leaves fall soon become colored is a common observa- 

 tion. The fact has significance for the farmer who has learned 

 by experience that, when hay that is down is caught in a rain, 

 more of the elements are washed from the cured hay than from 

 that more recently mowed and hence still partly green. 



Nature of Plant Food. — Besides oxygen, which is chiefly used 

 in respiration, various substances, such as water, sugar, acids, 

 salts, and carbon dioxide, enter the protoplasm where most of 

 them have some use related to the growth of the plant. But as to 

 whether or not all should be considered as plant foods, not all 

 students of plants agree; for, although all of these substances 

 have to undergo transformations in becoming cell structures, 

 some are more nearly ready for use than others. This may be 

 illustrated by comparing sugar with carbon dioxide and water. 

 In the leaves or wherever chlorophyll is present, carbon dioxide 

 and water have their elements dissociated and combined in such 

 a way as to form sugar which can be used directly for respiration 

 or by minor chemical changes be transformed into cell walls. 

 Thus sugar, since it is more nearly ready for use, may be called a 

 food and the carbon dioxide and water may be called elements from 

 which food is made. Likewise protein, which is closely related to 

 protoplasm, may be regarded as a food, while the mineral salts, 

 such as nitrates, phosphates, sulfates, etc., which are necessary in 

 the formation of proteins, may be regarded as the elements from 

 which food is made. Some investigators restrict the term plant 

 food to the more complex substances, such as sugars, starch, pro- 

 teins, fats, and amino acids, while others include some of the sim- 

 pler elements, especially the mineral salts. In this presentation 

 water, carbon dioxide, and the mineral salts are regarded as ele- 

 ments used in the formation of foods. 



