484 
VASCULAR CRYPTOGAMS, 
at all; it would appear rather, from the position of the vessels, that the axial fibro- 
vascular body consists only of the lower (inner) commencements of the foliar bundles, 
which are here densely crowded. In the same manner the basal disc-like woody 
body may consist only of the densely crowded commencements of the bundles of the 
roots. If this view is correct, the class of Dichotomeae presents two extremes, one 
in Psilotum, where the foliar development is small, and where there are, according to 
Nägeli, no foliar bundles, but the elongated stem forms a fibro-vascular bundle belong- 
ing to it only ; the other in Isoetes, where the short stem possesses no cauline fibro- 
vascular bundle, and only the strongly developed leaves have one each. The structure 
of the leaves of hoetes varies according as the species grow submerged in water, in 
marshes, or on dry ground. In the first case they are long and conical, penetrated by 
four air-cavities divided by septa into channels, with a weak fibro-vascular bundle in 
the axis of the organ, and the epidermis destitute of stomata ; in the second case they 
are similar, but provided with stomata and strands of hypodermal fibres ; in the third 
case the epidermis Is also provided with stomata, and the basal portions of the dead 
leaves (phyllopodes) form a firm black coat of mail round the stem. The ground- 
tissue is not separated from the single fibro-vascular bundle traversing the leaf by a 
bundle-sheath ; according to Russow it forms sclerenchyma under the epidermis which 
is usually colourless, in hoetes Hyjtrix, and dark brown sclerenchymatous strands which 
constitute most of the sheathing portion of the leaf. 
. Subsequent Continuous Growth in Thickness of the Stem. Outside the layer of clear 
tissue (phloem) which surrounds the central woody mass of the stem of Isoetes is a 
layer of meristematic cells by the activity of which the stem grows in thickness : it 
forms phloem-cells internally, thus adding to the fibro-vascular mass, and cortical paren- 
chymatous tissue on its outer side. The cortical tissue is formed much more rapidly 
than the fibro-vascular, and thus, in an old stem, the cortex is the preponderating tissue. 
This meristematic layer is evidently not analogous to the cambium of Dicotyledons and 
Conifers inasmuch as it forms fibro-vascular tissue on one side only, whereas in these 
groups of plants the cambium forms fibro-vascular tissues on both surfaces, xylem in- 
ternally and phloem externally. It is rather to be compared to the thickening-ring of 
DraccBna and other arborescent Liliaceae in which a continuous growth in thickness 
of the stem occurs. This view is supported by the fact that isolated bundles are 
occasionally formed by this meristematic layer in the stem of Isoetes. Thus Russow 
says that he found ' lying round the central woody mass of the stem of a robust 
specimen of Isoetes lacustris, but separated from it by five or six layers of cells repre- 
senting soft-bast, xylem bundles (consisting like the central xylem of shortly fusiform 
cells with irregular spiral thickenings) invested both on the outside and on the inside 
; by tabular cells; between these, the bundles radiating from the central woody mass 
to the older dead leaves are disposed.' No connexion could be traced between these 
bundles and either leaves or roots. 
A similar but much more considerable growth in thickness by means of a layer of 
meristem surrounding the axial fibro-vascular bundle has been recently shown by Pro- 
fessor Williamson to have occurred in the extinct Lepidodendra which are so commonly 
present in the Goal Measures, and which are evidently closely allied to the Selaginelleae. 
In these plants, however, if I rightly interpret Professor Williamson's account, it appears 
that a phellogen layer also existed at the periphery of the stem in correlation with the 
considerable growth of thickness resulting from the activity of the internal layer of 
meristem. These facts, taken in connexion with the probability of a growth in thickness 
of the stem of Botrychium, seem to indicate that this growth is generally wanting in the 
existing Vascular Cryptogams because they are less highly developed than their remote 
ancestors. 
[Professor W. C. Williamson has contributed the following note on the Carboniferous Lyco- 
podiacese : — ' The large and varied group of the Lycopodiaceous plants of the Coal Measures exhibits 
so many modifications that it is difficult to give a brief statement of their characteristic features. But 
