174 
Rendle. — On the Vesicular 
septa, which were apparently free or almost free from callus. 
Where this continuity was noticeable it was so obvious (Fig. 
5) that its apparent absence in the great majority of cases 
is probably due, not to difficulty of observation, but to the 
actual fact of its absence. In the external succulent leaf of 
the shoot in the centre of a spring-onion, where small nodules 
of callus were found on about half the transverse-septa ex- 
amined, — that is, where its formation was only just com- 
mencing, and where therefore, if anywhere, demonstration of 
continuity should be possible, — no indication of such could 
be found (cf. Fig. 6). The same was the case in very young 
stages where neither callus or pitting was visible. Not in- 
frequently in sections where no continuity of the contents of 
the vesicular cells was indicated, protoplasmic continuity 
between the parenchymatous cells was very well shown. 
In the green leaves pitting is less conspicuous and perfora- 
tion could never be made out. 
De Bary says the pits are not perforated, and from the 
present observations this w r ould seem to be the case in the 
great majority of instances. At any rate, we cannot trace any 
direct relation between their perforation or closure and the 
activity of the vital processes in the plant, such as obtains 
with sieve-tubes. All that can be said is, that in the succulent 
basal leaves of young onions, in the lower half only of which 
perforations were observed, continuity between the contents 
of adjacent members of the series sometimes occurs ; and it is 
here that our cells are most numerous and closest together, 
and that the longitudinal series are connected by cross-unions. 
The degree of protoplasmic continuity is evidently altogether 
insufficient to justify the application of the term ‘vessel’ to 
these structures. It is in fact no more extensive than that 
obtaining between parenchymatous cells. 
It is interesting from this point of view that callus-formation 
occurs where there is no evident pitting. Thus, in a young 
leaf of the shoot of an old onion which had germinated, out 
of six transverse walls examined, five showed no trace of 
callus, but the sixth, which was not pitted and scarcely thicker 
