CAUSES TENDING TO PRODUCE EXCESSIVE TRANSPIRATION. 387 



form, with numerous short, rigid branches, already described as being 

 sometimes assumed by the live oak, may have similar advantages. 



8. Plants with subterranean storage.— Bulbs, tubers, and other 

 strong, local thickenings of underground parts, which are greatly 

 developed in most arid regions, are not frequent among the dunes. 

 Cyperus grayi and 0. cylindricus have corm-like thickenings at the 

 bases of the stems. Woody, tuber-like swellings occur on the root- 

 stocks of the species of Smilax, and were also observed on the roots 

 in young plants of the live oak. The possession of a subterranean 

 food reservoir is unquestionably an advantage to a plant which is 

 liable to burial by the sand. 



9. Anniuds.— In very arid regions annual plants often avoid the 

 long period of drought by complei ing their course of life in a few weeks 

 of the growing season, and are therefore designated as "ephemeral." 1 

 On the strand annuals are numerous in species and individuals, but 

 there is no dry season, properly so called, to be guarded against, so 

 the different species reach the acme of their development at different 

 seasons. The annual life habit is probably more serviceable to dune 

 plants as a protection against being uprooted or buried. This is an 

 ever-present danger in the sand-strand formation, and one to which 

 species with a long life period are of course most liable. Besides 

 the radiantly growing species already enumerated, the following 

 annuals are frequent: Festuca octofiora, Aira yraecox (in woods 

 behind the dunes), Cenchrus tribuloides macrocephalus, Sarothra gen- 

 tianoides, and depauperate Erigeron canadensis, the last two pre- 

 ferring moist sand in the hollows. 



PROTECTION AGAINST EXCESSIVE TRANSPIRATION. 



A number of causes render it necessary that strand vegetation 

 should be guarded against too great loss of water by transpiration 

 from the leaves, just as plants of truly arid regions must be similarly 

 protected. The environmental factors which induce such peculiari- 

 ties of structure are, however, somewhat different in the two cases. 

 Here, as in most maritime regions, atmospheric humidity is abundant 

 and pretty equally distributed throughout the year. Furthermore, 

 there is no lack of water in the dune sands at a usual depth of only 

 15 to 30 centimeters (0 to \2 inches) below the surface, so that all 

 except the smaller (chiefly annual) herbaceous plants could readily 

 obtain an abundant supply at all times, were it not for another factor, 

 the presence in the soil of certain salts, particularly sodium chloride. 



The life conditions of the strand which are probably the most 

 effective causes of protective modifications of this nature may be 

 stated as follows: 



1. Strong insolation, and much light reflected from the surface of 

 the white sand. 



2. Heat, often intense, during most of the growing season. The 



1 Compare Volkeiis's El. der Agypt.-arab. Wuate, pp. 20, 40. 



