266 J. H. Priestley and L. M. Woffenden 
solute will be left deposited along the surface exposed to the air and 
thus a continuous deposit, very similar apparently in its nature to 
the original cuticle, appears upon the exposed surface. The presence 
of fatty substances in this layer is not surprising in view of the fact 
that Priestley and Armstead (21) found an ether soluble fatty sub¬ 
stance present in the sap rising from the vine root, whilst Hansteen- 
Cranner(io, li) has shown that fatty substances, both water soluble 
and water insoluble in nature, seem to form an invariable constituent 
of the walls in parenchymatous tissues. If these deposits at the wall 
surface are to be associated with such a drive of sap from the vascular 
strands, then the sub-stomatal cavity may be free from them where 
the stem endodermis is functional. Examination of the stomata 
upon the green cortical region of the stem of Camellia japonica cer¬ 
tainly failed to show any signs of such deposits, stainable in Sudan 
III, the contrast in comparison with the stomatal cavities of the 
plants previously mentioned being very striking. 
Douliot(6), in his extensive study of the distribution of the peri¬ 
derm in Dicotyledonous stems, draws attention to the marked effect 
of light upon the formation of periderm in the stem. In certain 
cases, Prunus spinosa L., Acer oblongus and Virgilia lutea , no periderm 
at all had formed upon the shady side of the stem when active 
periderm formation had occurred upon the sunny side. Some 
unpublished work by Miss R. Rea, M.Sc., in this laboratory, has 
drawn attention to the role played by sunlight in causing the rapid 
condensation of phellonic acid, one of the suberogenic acids (see 
Priestley (20)), but we are inclined to attribute this distribution of 
cork under lateral illumination to the more rapid evaporation from 
stomata in the sun and thus the earlier blocking of the substomatal 
apertures. 
The facts and observations quoted above lead to the con¬ 
clusion that the natural formation of cork at the surface of the stem 
follows as the result of the same general causal factors, previous 
blocking of sap flow, either at the endodermis or at the sub-stomatal 
region, followed by accumulation of sap to be followed again by the 
appearance of a meristematic phellogen. 
Summary 
i. Cork formation has been studied from a generalised stand¬ 
point arising out of a study of the effect of the sap contained within 
the vascular strand upon the meristematic activity of the tissue 
supplied by these strands. 
