Farmer . — On Is oetes lacustris , L . 47 
cells, which form the vascular strand, grow mainly in length, 
and remain of small diameter, the parenchymatous cells which 
immediately surround them as a double or triple layer divide 
rapidly in the transverse direction. The same is also true of 
the two rows at the periphery of the leaf, although in this 
respect they exhibit certain minor differences (see PL V. Fig. 
16). If the section has passed through the leaf in a direction 
other than that of the median plane, or one at right angles to 
it, the cells which occupy the space between the central strand 
and the periphery are seen to grow quite differently from the 
tissues on each side. They divide freely whilst in the merismatic 
region, but after emerging from it they scarcely increase in 
size at all and their nuclei are very small, a fact which is 
probably to be correlated with their arrested growth and 
extremely thin walls. Owing to this condition, and to the 
fact that the surrounding cells are largely increasing in length, 
the stationary cells are torn asunder in the longitudinal direc- 
tion, cohesion being first lost at the centre, whilst the peri- 
pheral cells remain permanently attached by their transverse 
walls : in this way are formed the diaphragms which cross the 
four air-chambers found in the mature leaf of the plant. The 
diaphragm is thinnest at its centre, i. e. is of double concave 
lens-shape, and consists usually of only one cell-layer at this 
point, becoming thicker at the edges where it is suddenly 
inserted ( sit venia verbo) on the outermost layer of the cells 
which were concerned in its formation (Fig. 16 in PL V). The 
earlier periclinal divisions which occur in the leaf are better 
seen in transverse than in longitudinal sections. 
' In transverse sections of young leaves in which the central 
strand has not as yet reached its final stage of differentiation, 
but which is, however, perfectly distinct, the outer cells of the 
leaf are seen to be increasing rapidly by periclinal or tangential 
divisions, which occur very regularly, and for the most part 
centrifugally. This is well illustrated by the photograph 
(PL VI. Fig. 22), in which the slight difference which exists 
between the development of the inner and the outer surfaces 
is already visible. It is only at a later stage that the diaphragm- 
