Histology of the Callus Tissue in Althaea rosea. 307 
substance in question. It has been stated both to be cuticular and 
pectic. The reactions given by the papillae in Althcea were certainly 
typical of neither substance. 
On staining with iodine, two regions could be distinguished in 
the larger papillae : a central blue spot and a colourless surrounding 
layer continuous with the stalk. The central spot gives all the 
reactions characteristic of the cell walls of Tropceolum seeds and 
one may therefore perhaps assume that it consists of hydrated 
cellulose or amyloid. The papillae are thus not homogeneous in 
structure, as has been asserted. The fact that they exhibit this 
differentiation may throw some light on their mode of origin. The 
substance of the outer part of the papilla and of the stalk is more 
difficult to identify. It is typically neither cutin nor pectin. 
Although it shews points of resemblance to both, micro-chemical 
tests only serve to exclude it from identity with any of the usual 
constituents of the cell-wall. Thus, the substance remains 
uncoloured with all iodine reagents, so that cellulose is absent. 
Recognised tests for mucilage and callose fail. There is no evidence 
of the presence of lignin. Almost every recognised test for pectic 
substances was tried, without success. In spite of the failure of the 
usual tests, the fact that the papillae underwent considerable 
swelling with reagents such as calcium chloride, potash and 
ammonia, coupled with Mangin’s very definite proofs of the 
presence of pectic bodies in the papillae of intercellular spaces and, 
in addition, the recognition of the large percentage of pectic 
substances present in the cell-walls of the normal pith, made it 
difficult to abandon entirely the idea that the substance in question 
was a modification of a pectic body. One fact supported this view. 
The papillae were not dissolved by dilute potash after treatment 
with concentrated hydrochloric acid and alcohol (1 : 3) for twenty- 
four hours, as is the case with ordinary pectic bodies. If, however, 
sections were placed in potassium chlorate and hydrochloric acid 
for a few hours, and then irrigated with dilute potash, not only did 
the ceils fall apart owing to the solution of the pectic bodies, but 
the papillae disappeared instantaneously. We may be dealing here 
with a hydrolysed product of a pectic body, the latter being 
reconstructed under the oxidising action of the chlorate. 
Similarly, although most of the tests for cuticular substances 
failed, there were a few reactions indicating that the substance in 
question is related to cutin. With Schulze’s macerating solution, 
and again with 25% chromic acid, the papillae separated from the 
