MODE ОҒ GROWTH OF BALLIA CALLITRICHA. 213 
are formed at all, or what special function they subserve, is not apparent; but the marvel 
is, that no sooner are they in existence than the openings seem to be thereupon again 
methodically plugged up. 
The terminal pits, with their corresponding stoppers belonging to the same cell, are 
as far as possible distant from each other, at the remote basal and upper ends of the 
cavity, whilst the pits appertaining to neighbouring cells (as is usual where pits occur in 
higher plants) are placed directly opposite to one another. Each pit is somewhat nar- 
rower at its inner end than at the mouth ; hence the two pits, viewed together, present an 
outline like that of a “pulley” in edge-view (Pl. XXVIII. figs. 11, 18). In the long 
and large cells of the main stems the pits are the largest, whilst they diminish gradually 
in size in the joints of the upper portions of the principal stems, or of the secondary or 
tertiary rachises, pari passu with their degree of tapering or attenuation (that is to say, 
with the decreasing size of the cells), until in the joints of the ultimate branches or 
pinne they become at last very minute, like mere granules or dots, but still quite per- 
ceptible (Pl. XXVIII. figs. 2 & 14). 
So far as can be learned by a close examination in joints which have not become 
injured, the primary membranes of the joint-cells seem to close up the bottoms of these 
adjacent pits; at least, a line can readily enough be discerned occupying that position 
(figs. 11, 16, 18). But frequently this appears, on examination with a high power, to 
have become obliterated ; not only so, but instances were noticeable where the contracted 
protoplasmie eontents of one of the joints (now quite colourless from the action of the 
spirit) had become from some cause thrust through the foramen (thus proving that these 
are real openings, not mere markings or even alterations of density or otherwise), the 
mass of contents (thus one half in one cell, the other in the other) roughly forming a 
figure-of-8 shape, owing to the constrietion caused by the narrowness of the aperture. 
Corresponding to this gradual diminution in the size of the pits of the stems and 
secondary branches onwards to the ultimate pinnules, there is not only a gradual diminu- 
tion in the size of the “stoppers,” as mentioned, but also a gradual modification of their 
figure. In those appertaining to the cells of the principal stems they are of a figure 
approximating to two thirds of a sphere; in the upper joints and in the smaller cells of 
the secondary and their lateral rachises they are of a figure nearly hemispherical (or 
kettledrum-shaped); and by degrees, as the cells diminish in size down to the smallest, 
they assume a nearly plano-convex figure, until finally, in the very smallest cells, they 
appear like mere dots, or brightish rounded or discoid granules. In all, it may be said, 
there is a more or less evident slightly projecting rim around the flat side, which rim 
often presents a slightly crenulated appearance (Pl. XXVIII. fig. 12). In those of the 
smaller dimensions and of a plano-convex figure, however, the margin appears to be 
somewhat “ bevelled off," and the crenulations forming an inner series. The convex or 
hemispherical surface is always quite smooth, and even glossy in appearance; ш dried 
examples it may become more or less broken or altered. The substance appears of a 
colourless, semitransparent consistence. It is the plane side which is apposed to the 
Opening of the pits, than which it is а good deal larger, closing it up like a lid—the 
SECOND SERIES.— BOTANY, VOL. I. ан 
