ROBINSON : SPINES OF FOUQUIERIA 49 
(Jour. de Bot. 13: 295. 1899) has described the leaves of the 
long branches as having sclerenchymatous cells prolonged from 
the sclerenchymatous layer beneath the epidermis of the stem, on 
the under side where the epidermis is in contact with the cortex. 
He considers this subepidermis as the outer part arising from the 
division of the cortex into two well-differentiated layers, and crit- 
icises Baillon (Hist. Pl. g: 242. 1888) for referring to the spines 
as leaves reduced to a midrib. Such a section as that shown in 
diagram in Fic. 9, where there are several layers of parenchyma- 
tous cells between the sclerenchyma (s) and the epidermis, shows 
that the sclerenchymatous cells arise not from a distinct outer layer 
of the cortex, but within the cortex. Like the prickles of the 
blackberry and rose, the spines of Fouguieria have no vascular tis- 
sue, but unlike them, the sclerenchymatous tissue arises from the 
cortex, not from the cuticle and the abscission layer is nearly at 
right angles with the axis of the stem instead of being parallel with 
it and in the surface of the stem. 
Various questions naturally arise in the study of such a struc- 
ture as the spines of the Houguiertaceae. How did these spines 
Originate? Was the stimulus external or internal which caused 
the original variation? Is Fouguieriaa genus that was separated 
from its parent form at a remote period, so that the intermediate 
forms between it and the other Pol/emoniales have become extinct, 
or is it a relatively new genus? Is it a stable form, or do the 
different plants vary in wide range ? 
Spinose processes are so characteristic of desert flora that 
the inference was long ago drawn that there is a close association 
between their structure and environment. The usual corollaries 
of spine development, reduction in surface and elaborate adapta- 
tions for water-holding, as well as the protective character of the 
spines themselves in warding off the attacks of animals, are such 
important factors in the maintenance of a genus in arid regions, 
that it is easy to reason that they are the outcome of influences in 
those localities external to the plant. 
Kerner, Henslow, and others of the older botanists have been 
inclined to emphasize the principle that “specific forms, on the 
whole, fit the places they have to live in,” rather than their inherent 
tendency to variation. No experiments have been performed upon 
