146 Gibson . — Contributions towards a Knowledge 
following pages will be found to contain a record of new facts, 
both anatomical and histological, which may serve to some 
extent as a starting-point for further investigation. 
Before entering on the detailed discussion of the species, it 
appears to me advisable to give a brief outline of the signifi- 
cation I attach to the terms employed in this paper. This 
seems the more necessary, as botanists who accept the 
terminology advanced by Van Tieghem as applied to the 
vascular system and its enveloping layers, and the morpho- 
logical views which such a terminology implies, are in the 
genus Selaginella brought face to face with a condition of 
things which in description requires very careful consi- 
deration. 
I have designated throughout the limiting layer of the 
stem epidermis , although in many respects, notably the 
entire absence of stomata, the lignification of the cell-walls 
and its developmental origin, it differs from the layer of cells 
usually bearing that name. The sclerotic tissue which in 
the great majority of the species lies immediately within the 
limiting layer, I have termed either stereome or hypodermis. 
It must be understood however that, inwardly, this layer, in 
most cases at all events, merges gradually into the non- 
sclerotic cells of the general cortex. There is no difficulty 
so far : it is when one begins to discuss the inner layers 
of the cortex, the trabecular tissue and the limiting layers of 
the vascular system, and again the morphological value of 
the vascular strands themselves, that one finds considerable 
confusion of nomenclature, not to say diversity of opinion. 
Vladescu (21) has advanced certain views as to the mode of 
origin of the tissues in question. This author, as already 
stated, finds that the segment-cell derived from the apical cell 
divides so as to form ultimately three cells, the innermost 
being the parent of the xylem-elements, the enveloping 
parenchyma and the phloem ; the outermost cell gives rise to 
the epidermis and the outer cortex, whilst the median cell 
gives origin to the tissue which encloses the phloem, the 
