308 Winifred Blackwell. 
membrane, but were not completely dissolved. Small rings and 
ellipses, representing the more or less spherical heads of the papillae 
after solution of their central spot remained floating in the solution. 
With iodine followed by concentrated sulphuric acid, the papillae 
turn brown without swelling, and again become separated from the 
membrane. A less resistant basal zone may possibly be present. 
It is interesting to note that by adjusting the concentration of the 
acid, the callus cells may be made to shrink, leaving behind, as they 
do so, a very fine colourless membrane to which the papillae are 
attached. In one case a thin brown cap, studded with papillae, floated 
off from the apical cell of the callus row. The degree of resistance 
of the papillae to chromic and sulphuric acids is similar to that offered 
by cuticular substances, but whether this is an adequate reason 
for relating their substance to cutin is somewhat doubtful. The 
identification of the substance with a hydrolysed product of pectic 
acid is based on more convincing evidence, and its resistance to 
acids, in this case, may possibly be explained by the circumstance 
that a pectic mucilage on exposure to air is apt to become altered 
into a tough horny substance of unknown composition. If this is 
so, the outer walls of the callus cells may have undergone partial 
hydrolysis and softening in preparation for subsequent fusion. The 
cellulose constituent has become amyloid, the pectic bodies have 
been hydrolysed to pectic mucilages. Since in this case fusion has 
not occurred between neighbouring cell rows, the hydrolysed 
products have become hardened on exposure to the air and have 
formed a resistant layer around the cells. It may be noted in this 
connection that Mangin, when working on similar protuberances 
in the intercellular spaces of plants, found reasons for regarding 
them as pectic in nature. We may perhaps correlate them with the 
fact that the air in the intercellular spaces is not rapidly renewed, 
and therefore the hardening process has not occurred. 
So far we have assumed that the resemblance of the unknown 
substance to cutin is accidental, and based only on its resistance to 
certain reagents. We have to face the possibility that the 
resemblance to cutin is actual and indicative of relationship. If 
this is the case, how has cutin been laid down in the papillae ? It 
was certainly not present, as such, in the walls of the original pith- 
cells. Although we know nothing certainly of the mode of origin of 
cutin in walls, it may be suggested that it has originated by the 
transformation of one of the known constituents of the wall. The 
young cell-walls of a typical plant consist of pectic and cellulose 
