Scott and Hill.—Structure of Isoetes Hystrix. 43 i 
possesses a ‘ caudex descendens,’ comparable to the tap-root 
of a Dicotyledon ; Hofmeister (185$, p. 159) lays great stress 
on the Dicotyledonous analogy. As a matter of fact, the 
downward growth is due to the cambium, which at the base 
of the stem appears to form secondary parenchyma only ; 
the wood which is added at the bottom of the stele, and 
which remains functional when the primary wood higher up 
is disorganized, is entirely built up by the sympodium of 
root-bases. There is thus no real similarity to the apical 
growth of a tap-root, though there is a physiological 
analogy, in so far as the Isoetes arrangement allows of the 
constant development of new roots at the base of the axis. 
The furrows owe their origin to the cambial growth being 
checked along the lines where the root-bases are attached, 
while it attains its maximum between them. 
The Leaves and Leaf-traces. 
The structure and development of the leaves of Isoetes , and 
of /. Hystrix in particular, have been admirably elucidated 
by Kruch (1890), whose accurate investigations have been 
confirmed in almost every point by our own observations 1 . 
A short statement of our results will therefore suffice. 
The mature leaf of Isoetes Hystrix , as seen in a transverse 
section of the lamina, i. e. above the sheathing base, has the 
structure represented diagrammatically in Plate XXIV, Fig. 18. 
The sectional form is roughly triangular, with the broad base 
of the triangle forming the upper or ventral surface. The 
mesophyll is traversed by four large intercellular spaces, 
alternating with which are four strands of mechanical fibres 
(Fig. 23), two at the ventrolateral corners, one at the middle 
of the ventral surface, and one at the dorsal angle. In this 
species the intercellular spaces are roofed in by the epidermis 
only ; the stomata are limited to those bands of the epidermis 
1 For a translation of a great part of the Italian text of Dr. Kruch’s paper, we 
are indebted to the kindness of Mr. L. A. Boodle, F.L.S. 
