Page:The Scientific Monthly vol. 3.djvu/49

 THE VARIABLE DESERT 43

result in marked diveraity in the water relations of the plant Bpeciea> even if the entire amount sank uniformly into the Boil. Beeidee thiB fundamental source of divereity, there are others.

The amount of water which may fall in each of the two periods, and, indeed, during the whole year, shows great annual variation. The winter and spring showera may be very local. There are not only con-

��Fig. 1. Id this flgure are Bbown the moDtbl; preclpltatloa for Bra IDdlTldual period! each of a bloloslol rear, beglDDlng witb NoTember and eadlng witb October. Tbe raina falling in NoTcmber and anbaegaent winter montha are cbleQy of algnldcance In the development of the winter or spring annuala and In repleniBblDg the reaerve of the cactt. Tbe ratna falling In October ma; complete the developmeat of the plsnta St Ihe lecond, mldaumnier, period of grmrth.

Kote that there la not onlj a dlTlaioD Into two dlltinct ralnr aeaaooa wblch recur annnallT, but tbat there Is great Tarlatlon in the BmoDDt and time of rainfall

The dark area la the average for fifteen rears aa given b; HacDongal. It la repeated under each fear to abow the extent of variation from the average.

�� �