STORAGE OF WATER 



195 



tion of food is in a manner similar to the winding of a weight, 

 and when the food or the protoplast is decomposed the resulting 

 readjustment of the elements keeps the vital activities going, just 

 as the falHng of a weight may set and keep in motion a train of 

 wheels, etc. More than 90 per cent, of the energy released in 

 respiration appears as heat, and the remaining energy probably 

 manifests itself for the most part in chemical reactions attendant 

 on growth, repair, secretion, and other constructive processes. 



The Storage of Water. — The need of plants for special 

 water-storage tissues is not so general as their need for tissues 

 in which to store food. Under 

 ordinary conditions water can be 

 freely taken in from the soil which 

 serves as the water reservoir for 

 plants. But plants of desert regions 

 or growing anywhere under xero- 

 phytic conditions (conditions making 

 water hard to get, as when it is 

 actually scarce or difficult to absorb 

 because of low temperature in the 

 substratum, or because there are 

 substances in solution in amounts 

 large enough to act as a poison or to 

 retard the osmotic inflow into the 

 roots) to which this reservoir is 

 denied or more or less inaccessible, have hit upon various com- 

 pensating devices. One of these is the water-storage tissue. 

 This is seen in its fullest development in succulent stems and 

 leaves. In Mesembryanthemum Forskalii of the Egyptian 

 desert approximately one-half of each succulent leaf is made up 

 of water-storage tissue (Fig. 106); and in epiphytic species of 

 Codonanthe growing on a dry substratum nearly three-fourths 

 of the fleshy leaf is occupied by cells devoted to the storage of 

 water (Fig. 81). The fleshy stems of cacti and some Euphor- 

 biaceae are largely composed of the same kind of tissue. In 

 the leaves of Ficus elastica the protoderm of the upper surface 



Fig. io6.- — Cross section of leaf 

 of Mesembryanthemum Forskalii 

 showing a large part of the leaf 

 devoted to the storage of water, a, 

 water-storage cells; b, chlorophyll- 

 bearing cells; c, crystal of calcium 

 oxalate. (After Schimper.) 



