344 THE FUTURE OF ARID LANDS 
term disease resistance. Plants may be susceptible or resistant to 
smuts, rusts, mildews, and so on. As research on plant diseases 
developed, it was found that there were races of many of the 
disease organisms and resistance to one race of rust, say, was no 
indication of the reaction to be encountered with another race. 
Disease resistance has many facets. 
Use of the term drought resistance is even more nebulous be- 
cause the term has not yet been satisfactorily defined. Water is 
an essential factor in photosynthesis since in the manufacture of 
food the carbon of carbon dioxide is united to the water brought 
up from the soil. This water remains in the plant until the food is 
oxidized or broken down, when it may be released. In addition, 
(1) water is an essential component of protoplasm, (2) it serves as 
a solvent for oxygen and carbon dioxide, (3) it has a high specific 
heat and absorbs much excess heat energy, (4) it aids in the 
transportation of raw materials and foods, and (5) it maintains 
turgor in living cells. In addition to satisfying all these needs the 
plant requires large amounts of water to replace that lost by 
bleeding, through the action of glands, by guttation, and by 
transpiration (55). 
Arid land species may be ephemerals, which are ‘‘drought- 
escaping”’ (60); succulents, which are a distinctive group of plants 
not only in structure but in metabolism and water economy (61); 
or “drought-enduring” species, whose cells can endure a severe 
reduction in water content for extended periods of time without 
serious injury, for example, creosote bush Larrea tridentata (59). 
Only plants of the latter group are truly drought resistant. 
Physiological Bases for Drought Resistance 
Attempts to explain drought resistance on a purely morpho- 
logical basis have proved inadequate, although certain structural 
features of plants undoubtedly aid in their survival in dry habi- 
tats (50). It seems clear that one of the basic factors in drought 
resistance of plants is a capacity of the cells to endure desiccation 
without irreparable injury. Desiccation of the protoplasm in itself 
does not result in death of the plant, according to Iljin (27, 28), 
but rather the mechanical disturbances such as pressure, stretch- 
ing, and tearing, which result from dehydration of the cell. 
