Robinson : Spines of Fouquieria 49 



fjour. de Hot. 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. PI. 9 : 242. 1888) for referring to the spines 

 as leaves reduced to a midrib. Such a section as that shown in 

 diagram in Fig. 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 Foiiquieria 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 Fmiquia-iaccae. How did these spines 

 orieinate ? Was the stimulus external or internal which caused 

 the original variation ? Is Fouquieria a genus that was separated 

 from its parent form at a remote period, so that the intermediate 

 forms between it and the other Poleinoniales 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 

 tho.se 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 



