200 Gibson. — Contributions towards a Knowledge 
The sieve-tubes have already been carefully described by 
Janczewski (17), and I have been able to confirm his account 
in every species in which I have paid special attention to 
these elements. Their walls are exceedingly delicate and 
show numerous but difficultly recognizable sieve-plates on 
such walls as abut on other sieve-tubes. Both lateral and 
end plates occur ; and it is possible to get a callus reaction 
from the thickenings which are not infrequent on the plates 
of large and old stems, e. g. .S'. Lobbi and S. Willdeuowii. 
The phloem-parenchyma, which forms a well-marked layer 
between the sieve-tubes and the xylem, consists of much 
elongated richly protoplasmic cells from one to four or more 
layers deep. Not infrequently isolated cells or groups of 
such are found amongst the sieve-tubes themselves. The 
distribution of these cells in relation to the sieve-tubes has 
already been described. The protoxylem-elements are 
tracheides and are spiral, annular or intermediate in character. 
The spiral elements may have one, two or three spiral 
thickenings, and considerable variety in the arrangement 
of the threads may be found. Save in two cases, the meta- 
xylem consists entirely of scalariform or reticulate tracheides, 
the former being by far the more common. Occasionally 
the tracheides are branched, especially near the origin of 
branches. In 5. oregana and S . rapestris , however, the 
tracheidal character is lost, and the elements are distinctly 
tracheae or cell-fusions partial or complete. I believe that 
this feature, though occurring but rarely in true ferns, has not 
hitherto been shown to occur in the Lycopodinae. I am 
not prepared to say that these species are exceptional in 
this respect, but so far as I have examined they appear to 
be so. 
In concluding the present paper, I would like to call atten- 
tion to the fact that, taking into account the anatomy of the 
stem only, we have been led to group together species widely 
separated by systematists, and conversely to separate species 
which agree strongly in external characters. Even in such 
