WATER BALANCE AND STORAGE i6i 



Brown (1912) found daily variations in the water content 

 of leaves of Physalis and Nicotiana of i to 6 per cent. 



In plants of a more xerophytic type different relations 

 are found. For Parkinsonia, a leguminous tree of the 

 Arizona deserts, a daily variation in the water content of as 

 much as 10 per cent, was found by E. B. Shreve (1914). 

 The small hard leaves do not show wilting, and we have 

 here a leaf that can suffer a loss of water, without serious 

 injury, and in the normal course of events, that would 

 certainly be fatal to the sunflower. Livingston found daily 

 variations of as much as 20 per cent, in desert plants. 



Water Storage. — In many plants the leaves or other 

 organs are provided with special water storage tissue which 

 very greatly increases the water balance on which they 

 work. A cross-section of the leaf of Tradescantia shows 

 the epiderm on each side to consist of huge colourless 

 cells between which lies a thin band of assimilating tissue. 

 In some species of Ficus the epiderm cells are large, in 

 others the epiderm is several layers thick. In Peperomia 

 there is a water-storage tissue many cells thick of epi- 

 dermal origin. In Rhizophora the storage tissue is sub- 

 epidermal. In the ice plant, Mesembryanthemum crystallinum, 

 the water is stored in modified hairs, huge bladder- cells 

 studding the leaf surface and giving the plant its curious 

 glistening appearance. Water storage tissue in the form of 

 enlarged epidermal cells is probably the most frequent ; 

 it is common in the orchids, perhaps even in some of our 

 native species. 



In succulents the water storage in the mesophyll of the 

 leaf, or in parenchymatous tissues of the stem, is extensive, 

 and these plants have a large water balance. The necessity 

 for this is imperative in such plants as the cactuses, which are 

 exposed to arid conditions for months on end, and have root 

 systems capable of absorbing water only when the surface 

 layers of the soil are moist. We have exact information for 

 the cactuses of Arizona from the work of Macdougal and 

 Spalding (1910). These plants are exposed to two dry 

 seasons of three or four months' duration, in early summer 



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