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 egarded 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, 
