264 Physiological Anatomy of the Vascular Plants, 
plant examined, staining with Sudan III. revealed the pre- 
sence of exceptional quantities of fats, especially in phloem 
and cortex. This examination brought to notice another 
anatomical feature which occurs in all these plants at a re- 
latively early stage. Transverse sections of stems stained 
with Sudan III. show, as a very conspicuous feature, a ring 
of cells just outside the phloem region, in which each cell 
is lined with a deeply staining lamella, formed of a deposit 
of suberin over the internal face of the wall. Such a cylinder 
of cells constitutes a secondary endodermis (Priestley and 
North — 3), and it is by no means of frequent occurrence in 
normal Angiosperm stems. 
Suberin is closely allied to cutin and is likewise derived 
from a number of fatty acids by oxidation and condensation 
(Priestley — 2). It is obvious, then, that the presence of a 
secondary endodermis in these plants of the peat habitat 
may be correlated with the excess of fats present in their 
tissues. The presence of this endodermis has marked effects 
on the growth of the plant. In the first place, as the imperme- 
able suberin layer forms and consolidates, the cortical tissues 
external to the endodermis are practically cut off from the 
supply of water and solutes moving within the vascular 
cylinder. As a result, these tissues become dry and withered, 
the stem in this region assumes a brown hue, begins to disinte- 
grate, and soon can be rubbed away, leaving exposed the 
cylinder of endodermis. Many observers must have noticed 
the fine dust raised during a tramp over the Calluna, Empet- 
rum and Erica of our moors. Such dust, usually ascribed 
to the chimneys of an industrial district, will be found on 
examination to consist in the main of the withered tissues 
of the plants themselves. During desiccation, the withered 
tissues undergo chemical changes which appear to make 
their walls much more resistant to subsequent decay, and 
much of the brown detritus powdering the surface of the peat 
is apparently due to this cause. 
Amongst the constituents of peat are also, of course, the 
remains of former generations of plants of similar anatomical 
structure. The tissues most resistant to decay are the fat 
impregnated tissues of cuticle and endodermis, and micro- 
scopic examination of peat reveals many such fragments. 
Indeed, if peat be submitted to saponification processes, a 
surprisingly large yield is obtained of the soaps of fatty 
acids (Priestley — 2) loc. cit. p. 23). 
Another result of the formation of a secondary endodermis 
is that in many species there appears at a later date a cork 
layer within the endodermis. The diagrams in text figure 
I . show the structural changes in the stem of Empetrum 
nigrum. A is a transverse section of the young stem showing 
N aturalisf 
