44 Farmer. —On Isoetes lacustris, L. 
only extreme forms of the same process, and all conceivable 
transitional stages may be met with in good preparations, that 
is, the radial wall of (i) may incline more or less from the truly 
radial direction, until it passes over into the form of (2). 
Around the apical meristem of the stem are situated the 
young leaves which originate as horseshoe- or crescent-like 
protuberances from the apical tissue. As is well known, they 
arise exclusively as outgrowths of the superficial layer of cells, 
and this point has been noticed as in some degree weakening 
the suggested resemblance to the Cycads, in which the peri- 
blem also enters into the formation of the leaf. It is however, 
I think, open to question how far this fact is of weight in either 
direction, for in Cycas the dermatogen and periblem of the 
stem lose their individuality in the actual apex, originating 
from a common layer of cells 1 . If however it were wished to 
press the resemblances, Isoetes might be regarded as an in- 
stance where this suppression of the individuality extended 
beyond the point of origin of the leaves, thus making this 
genus occupy a position as regards Cycas analogous to that 
of Cycas as regards the Abietineae. 
The Leaf and Ligule. — The leaf, after the rudiment is formed, 
by the luxuriant growth of the superficial cells, increases 
rapidly in size, and is at first completely enclosed by the 
sheath of an older leaf, which thus forms a kind of tube for its 
protection. The ligule is early discoverable at its base as a 
protuberant cell containing a nucleus of a very large size. It 
grows quickly, but never attains, in I. lacustris , to the propor- 
tions which it assumes in some other species, e.g. I. velata . 
In this latter plant it develops far more rapidly than the leaf 
which produces it, and, overtopping it, curves over the apex in 
such a manner as to protect the whole of the upper part of 
the leaf. The marginal cells of the expanded upper portion 
of the ligule are prolonged into short finger-like processes 
which not improbably contain a mucilaginous substance, and 
thus serve as additional means of protection against possible 
periods of dryness. In I. lacustris the ligule is, comparatively 
1 Strasburger, Coniferen, etc., pp. 335-6. 
