120 J. H. Priestley and Edith E. North 
more nearly a lignified than a cutinised or suberised membrane. 
These facts are of vital importance from the physiological stand¬ 
point and have been very fully re-examined. The reactions of the 
strip certainly suggest the presence of basal substances which have 
later been impregnated with other substances, but the problem appears 
to be more complicated than is indicated by Kroemer. 
(a) Impregnating Substances of the Casparian Strip. 
Evidence as to the nature of these substances may be obtained 
from the behaviour of the Casparian strip to staining reagents and by 
micro-chemical methods. This evidence has been admirably presented 
by Kroemer ( loc . cit. pp. 91-94) and may be summarised as follows. 
The staining reactions of the strip correspond very closely with 
those of lignified membranes; some of the reactions of suberised 
membranes are also given, but not more strongly than by many 
lignified membranes. Thus the Casparian strip stains with Sudan III 
but not more strongly than the xylem. The staining substances 
are easily removed by oxidising agents, eau de javelle, chromic 
acid, etc., less easily by boiling alkali; they are resistant to con¬ 
centrated pure sulphuric acid and render the whole strip resistant 
to this acid. 
From the micro-chemical examination of sections, it would there¬ 
fore appear that the strip is impregnated with lignin-like substances. 
We conclude, however, on the basis of further experiments, that the 
characteristic feature of the strip is rather the presence within it of 
derivatives of fats or fatty acids. 
Attention must be drawn to the conditions under which the strip 
is invariably formed. It appears in the layer of cells around the 
plerome cylinder, a layer which intervenes between the plerome and 
the first air spaces of the cortex. Within the plerome, at the level at 
which the Casparian strip is appearing, xylem and phloem are just 
beginning to differentiate. This differentiation involves great internal 
alteration in the future conducting elements and undoubtedly these 
alterations are accompanied by the giving up of organic solutes to the 
sap which may be assumed to percolate through the intervening walls 
up to the endodermal cylinder. At the outer surface of the cylinder 
this sap meets the air diffusing inwards from the intercellular spaces; 
thus we have the conditions arising which at the surface of the plant 
give at one time cutin, at another suberin. It is pointed out elsewhere 
(Priestley and Woffenden( 24 ), quoting in particular Tison( 29 ) and 
Devaux(8)), that the two processes of lignification and suberisation 
