20 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
(8) SUPERFICIAL SOIL LAYERS; ASSOCIATION OF ANNUALS. » 
Thus far, attention has been directed wholly to perennial species which, 
rooted in their places and remaining there for a period of years, constitute 
the more permanent and conspicuous elements of the vegetation. There 
are, however, a very considerable number of annuals which, at certain 
seasons of the year, become a highly characteristic and conspicuous 
constituent, and whose habits and distribution involve interesting and 
important problems. As in the case of the general association of per- 
ennial plants of the hill, we are here dealing with a more or less composite 
biological group, which, however, covers a wider area, and is perhaps 
even more influenced by distinctively local conditions. This, in its turn, 
will be studied to best advantage under several groups, which, though not 
sharply delimited, are sufficiently well defined. Taking the winter annuals 
first, two divisions based on physiological requirements are recognized. 
(@) SHADE-LOVING SPECIES. 
A good representative of this group is Parietaria debilis, which grows 
luxuriantly in the continuous shade of rocks on northern exposures. The 
water-content of this plant is very high, the stems having the shining 
pellucid appearance of the eastern clearweed, and there are no indications 
of specially developed means for the prevention of excessive transpira- 
tion. Though designated as shade-loving, because of its almost exclusive 
occurrence in the deep shade of rocks, there is, nevertheless, evidence 
‘that its growth here is conditioned by moisture, rather than lessened 
light intensity. It ventures here and there a little beyond the shade, 
and though strikingly modified in form and structure by exposure to 
full insolation for a part of the day, it is nevertheless capable in this 
situation of maturing its seeds. It is apparently a plant of essentially 
the same physiological requirements as the so-called shade-loving species 
of mesophytic forests, and in the one case as in the other jt may well be 
questioned whether, in the complex of physical factors necessarily in- 
volved, light intensity plays as important a réle as has been attributed 
to it. In any case, it is to be understood that the classification here 
adopted is retained chiefly for convenience in discussing the observed facts. 
A number of species of winter annuals grow luxuriantly on northern 
slopes and in the shade of rocks and bushes that are hardly met with on 
full southern exposures. Such are Bowlesia lobata and, to a less marked 
degree, species of Phacelia, Amsinckia (plate 11), and others. study of 
the habits of these plants, however, indicates, quite as plainly as in the 
preceding case, that the water relation is of primary importance. Bow- 
lesta, for example, which has grown thickly in the shade of a creosote- 
bush or palo verde, continues, when they are destroyed, to produce new 
crops year after year on the same ground, where the accumulation of 
humus insures a better water-supply. This and the other plants just 
