PLANTS OF THE SoTot REGION 629 
from the loose rocks among which it grows. This exposed part 
consists of numerous imbricated triangular-pyramidal horny lobes 
from the axils of the interior of which the flowers arise. These 
lobes display the assimilation tissue and are in addition storage 
organs. The significance of the horny exterior is that of an ex- 
ternal skeleton, which is the special need of a succulent body lying 
exposed to the crushing force of passing animals and shifting 
stones. This skeleton is so firm that the exposed part of the 
plant embracing the lobed portion retains its normal shape even 
after all the succulent part has disappeared from rotting, drying up 
or being eaten away by small animals. This chitin-like skeleton 
is a secretion of the epidermis which becomes many times as thick 
as the epidermal cells proper and is striated, showing the progres- 
sive thickenings (f. 7, cw). It is rather clear or like ground 
glass. Between the fissures and at the point of the lobes, this 
secretion is especially thick and horny. It is interesting to com- 
pare this tough epidermal secretion with that in the following type, 
Luphorbia antisyphilitica, which appears to arise in the same man- 
ner and is similarly striated, but is an easily frangible, soluble wax. 
The epidermal cells proper are very small and form two irreg- 
ular layers (f. 7, ef). This irregular arrangement together with 
the thick cellulose walls of these (c/) and the outermost assimila- 
tion cells, aids in binding the chitin-like layer more firmly. In 
spite of this, the whole skeletal covering becomes rather easily 
separated from the epidermal cells in the older parts (/. 7, 4). 
The stomatal opening is a very deep pit leading directly from 
the surface through the thick cuticle (f/ 7, sf) to the guard-cell 
apparatus, the cells of which are in a depression somewhat below 
the general level of the epidermal layers (f 7, g, 4, )- 
From the walls of the stomatal pit arise several stories of 
smaller outgrowths (f. 7, /am) stretched diaphragm-like across 
the pit, but perforated toward the center (a/) to allow the passage 
of gases. These perforations in the successive lamellae are so 
placed as to make a spiral zigzag passage for gases, from within 
Outward or conversely. Not infrequently, a section shows a 
nautilus-like spiral of fhese lamellae and their apertures. By this 
remarkable device is the movement of. gases hindered by a series 
‘of still-air chambers, and so excessive transpiration checked. 
