40 
Farmer . — On Isoetes lacustris , L, 
number of sections has however convinced me that Hof- 
meister’s view is more in accordance with the facts, and that • 
the axile bundle is really made up of leaf-traces, a view in 
which De Bary 1 also concurs. But in this case, like the alleged 
exogenous nature of the first root, to be considered subse- 
quently, there is represented just one of those transitional 
stages where distinctions lie rather in the mind of the investi- 
gator than in the actual object before him. In this particular 
instance it is exceedingly difficult to draw a limit between a 
possible cauline and a common bundle in the older stages, 
although in young plants it is perfectly obvious. The cause 
of the discrepancy existing between the accounts of the 
various writers possibly lies in the great difficulty, without 
a complete series of sections, in reconstructing the entire 
vascular system with any degree of accuracy, and the difficulty 
is further increased by the great number of leaves which arise 
at almost exactly the same level on the stem. 
About a year after germination, when the first few leaves 
are fully formed, the parenchyma around the vascular bundle 
of the stem (the pericycle of Van Tieghem) begins to divide 
periclinally to form the ‘cambium.’ The divisions extend 
around and above the axile bundle of the stem, but not so far 
as the youngest leaf-traces. And thus, whilst agreeing with 
Hegelmaier in the main features, I cannot but think that his 
description is misleading when he states that, whilst the cam- 
bium extends above the bundle in the form of a barrel, those 
cells which take part in the formation of a leaf-trace change 
their direction of division to one at right angles to that of the 
surrounding cells, in order to contribute to the trace. This 
statement is however only true of the appearance presented 
by older leaf-traces, which have become surrounded by, and 
enclosed in, the cambial zone. Originally, as I have said, the 
particular direction of their division is determined before the 
cells in their vicinity assume the freshly active merismatic 
condition. 
The leaf-trace originates in the division of a row of cells, in 
1 De Bary, Comp. Anat. p. 280 (Engl. Trans.) 
