34 
J. H. Priestley and J. Ewing 
the expense of the normal lateral growth of leaf and cortex. The 
collenchyma of the cortex is ill-developed, the angles of the stem 
opposite the main vascular bundles fail to develop, and the stem 
remains rounded in outline in cross section. 
In Pisum sativum L., and in Vida Faba, on continued growth in 
darkness, the Casparian strip may cease to form although growth 
still continues, the substances necessary for its formation (Priestley 
and North (17)) being possibly no longer available. Whenever the 
endodermis thus fails to form whilst growth still continues, its 
absence is indicated by the prompt appearance of these angles upon 
the stem and the greater development of the lateral leaf initials, the 
buds in their axils also frequently growing out. The same phenomenon 
may be seen in the potato shoot grown under normal conditions of 
light. Even under those conditions the first part of the shoot appear¬ 
ing from the tuber must develop in darkness, and so we find at the 
base of the normal green shoot a small region, usually a few centi¬ 
metres in height, possessing a primary endodermis, its presence being 
indicated superficially by the unopened nodes, by the roundness of 
the stem and by the absence of the prominent stem wings, charac¬ 
teristic of the upper region of the stem possessing a starch sheath. 
In view of the considerations advanced in an earlier paper 
(Priestley and Woffenden (19)) the capacity of the cortex of an etiolated 
stem to form cork might be expected to be extremely limited. Obser¬ 
vation suggests that the fatty substances available for the formation 
of a cuticle are less in amount, the cuticle being very thin in the etio¬ 
lated plant; and when this cuticle was removed from the stems of 
normal and etiolated potato plants, the injured surfaces examined 
in section three weeks later presented very different appearances. 
The outer cells were suberised in the normal manner in each case, 
but whilst in the green stem the phellogen had formed some six to 
eight layers of periderm by tangential division, in the etiolated plants 
tangential divisions had only taken place once or at most twice. If, 
however, cuts were made in the etiolated stems so as to penetrate 
the endodermis, reparation of the cut by means of a cork phellogen 
was very active indeed, and in the same period of time more tangen¬ 
tial divisions had occurred at the injured surfaces of the etiolated 
stems than at the surface of the green plants cut in a similar manner. 
Another interesting result was noticed in these experiments: when 
the cuts were made through the endodermis of an etiolated stem of 
the potato, even in the middle of an internode, after the cork had 
formed over the cut surface branch shoots frequently developed from 
