Scott and Hill.—Structure of Isoetes Hystrix. 419 
Fig. 2, from another, younger specimen, shows a transverse 
section passing immediately below the actual surface of the 
apex 1 . The large, somewhat triangular cell, with a large 
nucleus, is strongly suggestive of an apical cell, from which 
segments have been cut off. 
A third transverse series showed a similar predominant cell 
at the centre of the apex, towards which the surrounding 
meristem appeared to converge in radial series. 
Other specimens, cut transversely, gave no decisive results. 
Longitudinal series require the greatest care, in order to 
determine which is the median section. The apex is so flat 
that its form gives little or no guidance, and we were only 
able to satisfy ourselves as to the median plane, by counting 
the serial sections, and choosing the middle one, i. e. that 
which is equidistant from the youngest leaves on either side. 
We much doubt whether, in this species, the point could be 
determined without the aid of serial sections. 
Fig. 3 represents a longitudinal section through the centre 
of the growing-point. It shows a central cell (a) deeper than 
the rest, and with a large nucleus. On the left a segment 
appears to have been cut off, and then subdivided by a trans¬ 
verse wall. Fig. 4, from one of the adjacent sections, misses 
the large cell, but shows at s what appears to be a subdivided 
segment. 
It is no doubt possible that the various appearances de¬ 
scribed, pointing to the presence of a single apical cell, may 
be accidental, the large cell having no special relation to 
the general growth. With such a slow-growing apex, it is 
difficult, or perhaps even impossible, to arrive at certainty. 
It is, however, a somewhat unexpected fact, that whenever any 
definite arrangement could be detected, it appeared to favour 
the hypothesis of an apical cell. Our observations may at 
least serve to explain, if they cannot establish, the opinion 
of Hofmeister. 
The central cylinder of the stem is derived from a plerome- 
1 These are thin microtome-sections, about 6 /* in thickness, so that the section 
drawn shows the same cells as at the surface. 
F f 2 
