148 Gibson . — Contributions towards a Knowledge 
terms in the sense defined above). For instance, in the so- 
called bistelic species, such as S. Kraussiana , two such steles 
each with one protoxylem-group occur between the origin of 
branches, but at the origin of a branch these become united, 
i. e. gamostelic. On the other hand, one might consider the 
condition of the vascular tissue at the origin of a branch as 
monostelic, becoming between the origin of branches schizo- 
stelic. I have already pointed out that Van Tieghem himself 
seems to me not to be consistent in his use of the term stele 
in reference to Selaginella , and although I do not feel at all 
satisfied with the terminology, still in the absence of a genuine 
pericycle in the anatomical sense, I cannot see how the 
determination of what constitutes a true stele in this genus 
can be arrived at without reopening the subject of the stele 
as an anatomical unit — a matter foreign to the purpose of 
this paper. Personally I do "not think perfectly safe 
conclusions can be arrived at without careful examination 
of the primary embryonic axis — not the growing-point of 
a principal or secondary shoot of the adult. It is possible, 
for instance, that developmental evidence may be forth- 
coming to show that in the primary embryonic axis the stele 
is surrounded by a genuine pericycle — the erect secondary 
shoots becoming schizostelic or meristelic, the 'separate 
bundles or groups of bundles being surrounded by a layer of 
•specialized cortical tissue which may be considered as 
analogous to pericycle and endodermis. In any case, I am 
disposed to think that the morphological value of these 
sheathing layers is liable to be overrated. Professor Bower 
informs me that an endodermis is well marked in the rhizome 
of Helminthostachys , that it is not continuous either as 
a general or special sheath in Botrychium Lunaria , although 
present at the periphery of the bundles of the axis. Ophio - 
glossum vulgatum and O. reticulatum , he also informs me, 
have doubtfully an endodermis, and 0 . Bergianum certainly 
wants it. Such cases as these, together with others such as 
that of Equisetum , make one chary of attaching very great 
weight to the presence or absence of sheaths, and in the 
