PLANT ECOLOGY AND FLOMSTICS OP SALTON SINK. 97 



to which to attribute the presence or absence of these plants; but in an equihbrium of deli- 

 cately balanced conditions a slight cause may have a determining effect. Be the cause 

 what it may, the fact remains that the Sink is below the altitudinal limit of succulent plants. 



Induments of various kinds, and aromatic exhalations, are characteristic of xerophytic 

 plants and are often extensively developed; but here, where conditions of aridity are extreme, 

 protections of this nature have not been evolved to a marked degree. Larrea has a highly 

 varnished surface and a strong odor, but it properly belongs to a higher zone. The Atriplices 

 have a scurfy coating, and other shrubs, such as Franseria dumosa, are moderately pubescent 

 or hirsute. Among herbs, annual and perennial, this character is more pronounced, as exem- 

 plified in Psalhyrotes ramosissima, Coldenia plicata, and Encelia eriocephala. None of the 

 trees have developed characters of this kind beyond a very slight degree. 



Nor are there in the flora of the Sink, with two exceptions, plants which possess any 

 form of storage root, a development so often found in arid regions. The exceptions are 

 Cucurbita palmata, which has moderately large tubers, and Hesperocallis undulata, which 

 has a deep-seated bulb. The first is not uncommon in most parts of the Sink, but is more 

 abundant in the canons of the adjacent mountains; while the latter, although common 

 in the southeastern borders of the Californian deserts, has as yet been detected in the Sink 

 only at Mecca. The results of the arid conditions under which the xerophytes of the 

 Sink maintain their existence is doubtless manifested in important anatomical and physio- 

 logical modifications, but these are not here considered; only the evident exterior adapta- 

 tions of the plant organs can be noted. The agents by which these are produced are the 

 excessive insolation in a region of ever-cloudless skies and the great evaporating effect of 

 a dry and windy atmosphere on plants inadequately supplied with water. 



It is to these causes that the distinctive aspect of the desert flora is due, so different 

 from shrubs of more favored regions. The condensed growth of the plants, affording the 

 maximum of shade and protection, has been already adverted to. The same causes have 

 produced the small, narrow leaves, sparse and often early deciduous, so that some are 

 leafless for much of the year, which is so common a feature of the desert flora. Atriplex 

 hymenelytra is an exception in this respect, but its broader leaves are thickened and the 

 epidermis permits so little transpiration that branches broken off are weeks in becoming 

 dry. The prevailing gra3mess of the desert vegetation is a response of the isolated and 

 exposed plants to the intense sunlight which pours upon them. It affords to the chloro- 

 plasts a needed shade. 



Considerations such as these are commonplaces in treating of desert floras. A study 

 less hackneyed, more difficult of prosecution, but promising important results, relates to 

 the character and the extent of the ligneous and fibrous root systems of the desert xero- 

 phytes. An example of the method and the value of such investigations is offered by 

 Cannon's "Root Habits of Desert Plants. "' 



To such a study the flora of the Sink offers an inviting and an untouched field. The 

 soils differ widely in their composition and texture, and consequently in their capacity 

 for admitting and retaimng water, in the depth to which it may descend, or from which 

 it may be raised by capillarity, and in the facility by which they may be penetrated by 

 the roots of plants. While experimental proof is wanting, it is certain that these factors 

 are largely those which determine the distribution of the xerophytic flora. The revelations 

 made by the washing away of bluff banks show that even in compact soils roots penetrate 

 to great depths. 



There are certain anomalies to which such a study would probably furnish the key. 

 The two species of Prosopis have an abundant, although finely divided, foliage, and they 

 flourish best and attain then- greatest size in damp soil, yet they are to be seen m very arid 

 soil, and P. glandulosa seems unaffected even when b uried to the tips of its branches in 



' Carnegie Institution Publication No. 131, pp. 59-61, 96, pi. 23, 1911. 



