HISTORY AND PROBLEMS 11 
of palisade tissue and small intercellular spaces. The spongy 
tissue is reduced. 
The drought-resistant succulents show the same structures for 
preventing water loss but also have impounded water in roots, 
stem, or leaf, or all of these, and are able to continue growth when 
no water is available in the soil. They rapidly absorb water from 
summer showers, have an extensive superficial root system, are 
often globular or columnar, and have a large volume as compared 
with the surface. They resist drought by impounding water in the 
plant body. 
The physiological adjustments are equally striking. Unlike 
the mesophytes, the leaf cells of xerophytes do not wilt when they 
lose water. The leaves are either dropped or, if they remain on 
the stem, they may lose as much as 40 to 80% of their water and 
still recover when water is again available. The reason for this 
recovery is that, unlike the mesophytes, the dehydration does 
not cause coagulation of the protoplasm or rupture of the protoplast 
to produce permanent and irreversible harm. A possible explana- 
tion of this condition is found in the high osmotic pressure of from 
20 to 7§ atmospheres and possibly the effect of hydrophylic col- 
loids and sugars on the protoplast. 
The ecological means by which plants meet drought conditions 
and grow in regions where droughts occur are of four types (8). 
1. Drought escaping plants grow only where or when there is no 
drought. Here belong the summer and winter annuals of our South- 
west such as the six-week grasses and the ‘‘ashab”’ of the Sahara. 
They usually grow under mesophytic conditions and except in the 
seed ripening and seed condition, do not encounter drought. They 
live through the hot, dry, drought period in the seed stage, a 
dormant stage similar to estivation. 
2. Drought evading plants are plants economical in the use of 
the limited soil moisture supply. This is accomplished by wide 
spacing, by keeping the plants small with a small leaf surface and 
a small amount of annual growth. The root systems are propor- 
tionally very large. These plants are often very efficient in the use 
of water. Under like conditions plants may require as much as from 
300 pounds to 2200 pounds of water to produce 1 pound of dry 
