SCANTINESS OF DESERT VEGETATION. 43 



of the region is most conspicuously reflected. Size, spacing, and form 

 of desert shrubs may be taken as their general characteristics. 



The height of Larrea tridentata, the largest of the desert shrubs, is 

 commonly from 1 to 1.5 meters, while that of Frameria dumosa is 

 about 0.3 meter. Between these limits the height of nearly all the 

 other desert species ranges. In situations of extreme aridity the scale 

 is slightly decreased, and in soil unusually well adapted to the needs 

 of the vegetation the size of the plants somewhat exceeds these limits. 

 The scantiness of the desert vegetation possesses more signilicance 

 than has ever been attributed to it. Except in rare instances the 

 individual shrub is separated from its nearest neighbor by a distance 

 of several meters. Never do they stand so close together as to crowd or 

 shade each other. The distance from plant to plant varies with the 

 aridity of the soil and is determined by it. Only so many individuals 

 grow over a given area as can be supplied with moisture from it. In 

 a humid climate the individual plants, whether trees, shrubs, or herbs, 

 grow close together and by reason of the shade thus produced tend to 

 deprive each other of light and of heat. The result is an active com- 

 petition of both individuals and species for these, necessaries of life. 

 Between the separated individuals of the desert, however, no shade is 

 produced, and they carry on, therefore, no struggle against each other 

 for heat and light. 



It is evident therefore that desert shrubs essentially present in their 

 environment the anomaly of a struggle for existence, not against other 

 plants, but against nonorganic physical forces alone. This fact makes 

 the study of their adaptations especially interesting and instructive, 

 for one element in the usual complexity of environment is removed, and 

 we are able to perceive the simple influence of climatic and soil con- 

 ditions. 



The form of desert shrubs is the immediate outcome of their spacing. 

 Surrounded on all sides by light, they develop in every direction 

 equally. The resultant form is rounded or globular, a type from which 

 there is almost no departure. In a shaded situation a light-loving 

 plant, in order to obtain sufficient light, must grow tall enough to reach 

 above the shading body, and in doing so necessarily loses, or fails to 

 to develop, a rounded outline. A creeping shrub or a cone shaped 

 shrub is unknown in the desert. Many species have a well-defined 

 trunk and present the appearance of miniature trees, while many others 

 have only the group of stems characteristic of ordinary shrubs. Pen- 

 cephyllum sehottii is typical of the first of these forms, Larrea tridentata 

 of the second. These two shrubs exhibit also a diversity in individual 

 maturity which is worthy of record. A vigorous specimen of Veucepliyl- 

 lum commonly attains a height of 1.25 meters and has a trunk 7.5 to 10 

 cm. in diameter. Sooner or later, becoming incapable of carrying on 

 the functions of vitality, it dies. Such a termination of life is common 

 to all our ordinary forest trees. They have no provision for indefinitely 



