424 Scott and Hill.—Structure of Isoetes Hystrix. 
such as obtained in Sigillaria and most Lepidodendreae of 
which the structure is known. On the whole, however, the 
process in Isoetes more nearly resembles that occurring in 
Lepidophloios fuliginosus , the most irregular of the Lepido¬ 
dendreae in its secondary growth. 
The cambium, in I. Hystrix , arising in the tissue -just out¬ 
side the xylem-cylinder, continues its activity indefinitely, 
producing parenchyma and phloem, and a variable amount 
of secondary xylem, on its inner side, and secondary cortical 
parenchyma only, towards the exterior. Where the cambium 
is at first a normal one, with phloem on its outer side, its 
activity is of short duration, and it is immediately replaced 
by a new generative layer, arising further to the exterior. 
Usually the cambium, from its first origin onwards, is anoma¬ 
lous, in so far as it produces phloem towards the interior 
exclusively. There is no regular formation of successive 
cambial layers : the same layer may apparently continue active 
throughout the secondary growth. In two cases, however, 
a new cambium, internal to the first, was observed, arising 
by the division of secondary parenchymatous cells just outside 
the primary wood. As this internal cambium did not exist 
nearer the apex, the presumption is that it was really of later 
origin than the more external zone. The activity of the two 
cambial layers had produced some crushing of the elements 
between them. The inner cambium, like the outer, may 
produce all three tissues—phloem, wood, and parenchyma— 
on its inner face. 
The active cambial layer can be readily identified by its 
narrow, tabular cells, containing dense protoplasm, and each 
with a large nucleus. In this species, at any rate, the cambial 
cells do not contain starch, as Wilson Smith maintains to be 
the case in the species investigated by him (p. 227). 
The alternating, though not very regular zones, of paren¬ 
chyma and phloem, are a constant feature of the intracambial 
tissue. In an old stem we counted from eight to ten layers of 
each. 
On the other hand the development of the secondary wood 
