DISSEMINATION OF SEEDS. 45 



SPECIAL ADAPTATIONS. 



In turning now to examine the minuter structural characters of the 

 desert flora, one finds among the annuals little that is remarkable. 

 Their leaf surface is well developed, many have soft herbaceous foli- 

 age, none have thick succulent leaves, and comparatively few have 

 marked modifications to prevent rapid transpiration; yet some adaptive 

 characters may be detected. A few species are canescent with a close 

 covering of hairs {Dalea mollis, Plantago patag&nica gnaphnloides, 

 Psathyrotes annua, and others), and many {as Abronia villosa, Mohavea 

 breviflora, and Phacelia crenulata) bear viscid- glandular hairs. 



It is in the shrubs, however, which are subjected to all the seasonal 

 changes of many years, that the most marked modifications occur. 



There are several important stages in the life histories of plants, at 

 any one of which, if an adverse or peculiar environment is presented, 

 special adaptations are required in order to insure the continued effi- 

 cient existence of the species. Perhaps the most important of these 

 are the periods of reproduction, dissemination, hibernation, germiua- 

 tion, and growth. 



The process of reproduction seems to present in these plants about 

 the same problems as in those of an ordinary climate. The customary 

 methods of pollination, by insects (as in Opimtia eehinocarpa) and by 

 the wind (as in Atriplex hymenelytra), are found here as in other 

 regions. A cursory view of these matters did not show that wind-pol- 

 lination and self-fertilization were unusually prominent. Insects ap- 

 pear to be sufficiently numerous to accomplish all necessary floral visits. 



The dissemination of seeds and fruits presents a wide range of adap- 

 tations. The wind is enabled to distribute the achenia of many com- 

 posite (Aster mohavensis, Tetradymia spinosa, etc.) by reason of their 

 finely divided pappus; the fruits of Larrea and Eurotia by the long 

 divergent hairs on their surfaces; those of Atriplex^ Grayia, and Sar- 

 cobatus by lamelliform enlargements of the involucre; and those of 

 Salazaria by a bladdery inflated calyx. In none of these cases is the 

 fruit buoyant enough to remain suspended in the air, but it is sufficiently 

 light to be blown along the surface of the ground by an ordinary wind. 

 In Eriogonum trichopes the stem breaks off at the base and the whole 

 plant is blown over the desert as a tumble-weed. The fruits of Krame- 

 ria canescens, Franseria dumosa, and Petalonyx thurberi are provided 

 with barbed bristles, by which they may catch in the fur of animals and 

 be transported to long distances. Only a few species produce fleshy 

 fruits (Lycium andersonii, Mamillaria tetrancistra, and a few others). 

 The fruits of Opuntia, which in less arid regions are often fleshy, are here 

 all dry; and, in general, it is evident that the tendency is against the 

 evolution of plants with pulpy fruits. This tendency undoubtedly re- 

 sults, not from any insufficiency of such a means of dissemination, but 

 from the inherent difficulty of producing fleshy structures in so dry a 



