8 



THE AMERICAN BOTANIST 



it is as useless to plants as if it did not exist. Thus we find the 

 plants behaving as they do in the desert, growing when water 

 is available and dying or going into a resting period at the 

 approach of drought. The perennial plants protect them- 

 selves from completely drying up in winter by dropping their 

 leaves and shielding their tender young cells by bark, bud- 

 scales, down and varnish. Any spot from which the 

 water rapidly escapes after a rain will be xerophytic. 

 Included in this catagory are sand dunes, cliffs and the trunks 

 of trees. The prevalence of salt or other substances in the 

 soil-water may also cause xerophytic conditions by hindering 

 the flow of water into the plant. Many bogs thus present 

 really desert conditions to the plants inhabiting them. Like 

 the frozen soil of winter, the bogs are not physically dry, but, 

 what amounts to the same thing, they are physiologicall}^ dry. 



The characteristics which make a plant most successful 

 as a xerophyte may be placed in three groups : they must have 

 means for rapidly absorbing moisture; they must have some 

 way of storing the moisture absorbed ; and they must possess 

 means for preventing this stored moisture from escaping into 

 the air again. An extensive root system is usually the means 

 of absorbing moisture, though some xerophytes, when epiphy- 

 tic, may absorb through their leaves. Storage of water may 

 occur in regular cisterns formed by the overlapping bases of 

 the leaves or the leaves themselves may become cisterns. Illustra- 

 tions of this may be found in the wild pines and pitcher plants of 

 tropical regions and even our own pitcher plants are now 

 regarded as xerophytes of this type. Water may also be 

 stored in tuber-like receptacles as in some ferns and orchids, 

 in underground roots and stems, but probably the more 

 usual method of storag'e is in certain cells of the leaf, called 

 water storage cells. These may form a sort of water blanket 

 enveloping the green parts of the leaf or they may be in the 

 interior of the leaf surrounded by the green tissue. 



