Scott and Hilt.—Structure of Isoetes Hystrix. 425 
is extremely variable, though it is always present to a greater 
or less extent. In one stem of fair size we found, as already 
mentioned, no other secondary tracheides than the few occurring 
immediately outside the primary wood. In other stems, one 
or more irregular bands of secondary wood are differentiated 
here and there between the phloem-zones (Fig. 10). Sometimes 
the secondary tracheides are isolated (e. g. Fig. 9); in other cases 
they form considerable aggregations, as shown in Fig. 11, 
from a specimen in which there was an almost continuous 
band of secondary wood round the stem. There is no doubt 
that the elements of the secondary, like that of the primary 
wood, are true tracheides, which not only acquire lignified 
thickenings on their cell-walls, but lose their living contents. 
The statement made in the case of /. lacustris , that these 
elements contain protoplasm and starch, certainly does not 
hold good for I. Hystrix (Farmer, 1890, p. 42 ; Campbell, 1895, 
p. 291). The secondary tracheides, where they occur, are 
perfectly sharply differentiated, and agree in all important 
respects with those of the primary wood. 
The extracambial secondary cortex does not attain such a 
great development in I. Hystrix , where the primary cortex 
is not exfoliated, as appears to be the case in some other 
species. The thickness may even be less than that of the 
intracambial secondary tissue, especially low down the stem. 
There are some indications that the secondary cortical cells 
may undergo further divisions after they are cut off by the 
cambium. 
Thus the secondary growth of the Isoetes stem is on the 
whole anomalous, if we take the typical Gymnospermous or 
Dicotyledonous stem, or that of most of the Lepidodendreae, 
as the standard of comparison. Its peculiarity consists in the 
fact that as a rule, though not invariably, the true phloem, as 
well as the secondary xylem, is formed on the inner side of the 
cambium, accompanied by secondary ground-tissue, while on 
the outer side, ground-tissue alone is produced. The mode of 
growth is thus roughly comparable to that of the Monocotyle¬ 
dons, such as Dracaena ) Tamils Arisiea,wlth secondary tissue- 
