175 
The palo verde (Parhinsonia microphylla) is chiefly character- 
ized by its green smooth bark and numerous but usually leafless 
twigs. Leaves occur upon young plants, or upon new shoots, 
but are seldom noticeable. It is evident that the leaf function is 
transferred to the superficial green tissue of the stems. In form 
it suggests a well trimmed small orchard tree, although in detail 
it has its own features, not easily described by the pen, the most 
striking of which is the great number of slender tapering, usually 
Fic, 28. Palo verde (Parkinsonia microphylla), 
spinose twigs, which render the outline of the tree soft and ill 
defined (Fig. 28). 
n addition to these more dominant elements, there is a varied 
flora of smaller shrubs of the genera Lycium, Acacia, Celtis, 
Covillea or creosote bush, which after the rains makes a splendid 
show of bright green foliage and yellow flowers, tree opuntia, 
prickly pears, barrel cacti, mamillarias, all these and many 
others, all of which offer material for research. 
