39S Farmer . — On the Morphology and 
separation is almost always at the junction of the ends of 
two older mother-cells, that is, of cells which have divided 
a number of times transversely, so as to have given rise to 
a number of daughter-cells. This fact has some bearing on 
the problems as to the nature of the causes which effect the 
separation of cells in general, and in this case the cause is 
probably to be looked for in the degeneration of the middle 
lamella, and this explains why the separation commonly 
occurs between the points of contact of older cells, since such 
degenerating changes have been longer in operation at these 
places. In PI. XXV, Fig. 5, is seen an example of such a 
separation, and the appearance there represented is highly 
characteristic of all similar occurrences in this plant. A 
small drop of gelatinous-looking substance reaches from the 
tip of one cell to the other, exactly as one would expect on 
the explanation just advanced. But Hedera is not a very 
suitable object in which to study the phenomena of cell- 
separation, which is presented in a far clearer manner in 
Crataegus Oxyacantha. If half-ripe fruits of this plant be 
examined, the cell-walls are seen to be very thick, and even 
without staining a striking differentiation of the walls into layers 
is quite obvious. This is of such a nature that the portion 
of cell-wall which abuts on the cell-contents is clear and dis- 
tinct, with shallow pits, and resembles that of ordinary paren- 
chymatous elements ; but the portion which lies between two 
contiguous cells is of a brownish colour, and resembles gum in 
some of its characters. It can hardly be termed mucilaginous, 
as it scarcely swells at all in water, though potash and strong 
acids cause a decided increase in bulk to take place. The 
whole layers of cell-wall are very clearly differentiated by 
staining with haematoxylin, Hanstein’s violet or, best of all, 
with Schultz’s solution. At a somewhat later stage irregular 
cracks appear in the gum-like middle lamella, and finally 
intercellular spaces of a large size are formed, and in many 
cases the slimy substance of the degenerating portion of the 
wall can be seen stretching across in strands which fuse with 
the mass lining the spaces (see PI. XXV, Fig. 1 7 )- As degener- 
