Histology of the Callus Tissue in Althaea rosea. 309 
bodies. Where these are exposed— e.g., epidermal cells, walls of 
pollen-grains, etc.—cuticularisation frequently occurs. Mangin 
noticed that in the walls of Fungi where pectic bodies were 
constantly absent, cuticularisation never occurred; from this fact, 
supported by many other observations— e.g. the tendency of pectic 
substance to accumulate on the outside of the cell—Mangin was 
led to suggest that cutin was formed by the transformation of the 
pectic constituents of the wall. This seems a possible explanation 
of the present difficulties. If the hypothesis is correct, we are 
dealing with different transition stages in a series of plastic substances 
in which pectic bodies and cutin are the first and last terms. The 
composition of the wall is determined by the degree of exposure to 
the air. Middle lamellae are therefore definitely pectic in nature. 
The walls lining intercellular spaces, whether papillae are present 
or not, are generally pectic, although they give cuticular reactions 
in some cases, especially when the spaces are large, as in the 
Nymphaeaceae ; (cf. also Jost on papillae of the intercellular spaces 
of the pneumatophores of Phcenix). The papillae on the walls 
exposed by wounding are more definitely related to cutin. The 
outer walls of epidermal cells are typically cuticular. 
Since the pectic substance of the middle lamella is chiefly in 
the form of calcium pectate, if cutin be the final product of its 
transformation, calcium must necessarily be expelled at some stage, 
It is of interest to note that a large number of calcium oxalate 
crystals are present both within the cells of the callus-tissue and on 
the tips of the cell-rows. Comparison may be made with the leaves 
of such a plant as Ficus, the epidermis of which is characterised by 
a thick cuticle, and by the localisation of special calcium carbonate 
reserve cells among the normal ones. 
It is difficult to see of what use to the plant the papillae of the 
intercellular spaces may be. Mangin has suggested that their 
significance is not teleological, but depends on a purely mechanical 
flowing together into little lumps of the partially lignified substance 
of the middle lamella in the formation of the intercellular spaces. 
Similarly, the papillae on the walls of callus cells may be formed 
mechanically during the softening of the walls prior to fusion, the 
hydrolysed pectic substance collecting, as is usual, towards the 
outside of the wall. Where fusion of the cell-rows has not occurred, 
the pectic substance on exposure to the air has undergone 
“ hardening.” Where a parenchyma has been formed, only the 
original end walls of the cell-rows remain exposed. They were 
