266 Physiological Anatomy of the Vascular Plants. 
the endodermis (e) ; B shows a later stage with the layer 
of cork (c) arising from the cork cambium (c.c.) within the 
endodermis ; C shows further development of the cork. 
The endodermis has now disappeared, having been burst 
asunder by the growth of secondary wood and cork, and 
rubbed away, together with the cortical tissues. In B and 
C can also be seen the heavy deposit of fat in the phloem (ph) . 
It may be noted that when the endodermis disappears, its 
duties as a protective layer will be taken over by the cork 
layers formed within it. The exact relations of endodermis 
cork, and other tissues, vary from plant to plant, and in the 
Appendix will be found brief notes relating to plants from 
Yorkshire peat habitats which have so far been examined. 
The point so far emphasized is that, in the peat plants 
examined, an obvious connection may be traced between the 
thick cuticle and heavy suberin lamella of the secondary 
endodermis, and the excessive amount of fat present in the 
tissues. It is now proposed to suggest a reason which seems 
adequate to explain the presence of this excessive fat deposit. 
The roots of peat plants are growing in a soil which is essen- 
tially deficient in oxygen. Observations seem unanimous 
on this point. If a root grows actively under such anaerobic 
conditions, one of the most effective sources of energy for its 
constructive metabolism would be found in the conversion 
of carbohydrate into fatty acid with elimination of some C 0 2 . 
Such chemical changes are known to occur within the living 
organism (Leathes — i, loc. cit., p. 107), and in this Depart- 
ment considerable evidence has been found for such activity 
in the root apex. 
Behind the growing region these fatty acids seem either 
to be deposited upon the walls as insoluble calcium soaps, 
or to be carried up with the sap in the vascular cylinder as 
soluble soaps of potassium or sodium, or as free fatty acids. 
If, then, this metabolic change is more predominant in 
roots growing in peaty soil, greater quantities of fatty acid 
will be formed here than in normal roots. Peat soil is notor- 
iously deficient in calcium, so that we may anticipate that 
the acids are not precipitated in the root, but move upwards 
with the sap supply to leaf and stem. Here they accumulate 
at the surface in the abnormally thick cuticle, and later are 
deposited within the stem as the suberin layers of the second- 
ary endodermis and cork. This explanation, if supported 
by further investigation, would incidentally throw some 
light upon the ‘ calciphobe ’ nature of the plants of a peat 
habitat, a subject which will be treated separately in another 
note from this Department on the significance of the 
Na 
Ca/-^ ratio to the plant. 
JK. 
Naturalist 
