The Structure of the Aerial Shoots of Psilotum 
flaccidum. Wall. 
BY 
W. STILES, B.A. 
Assistant-Lecturer in Botany in the University of Leeds ; formerly Scholar of Emmanuel 
College , Cambridge. 
With Plate XXV. 
Introduction. 
HE genus Psilotum is generally regarded as comprising two species : 
Psilotum triquetrum , Sw., and P. flaccidum, Wall. The two species 
differ chiefly in the form of the aerial stem ; in P. triquetrum this is multi- 
angular in the lower parts and triquetrous in the ultimate branches, while 
in P. flaccidum the lower part is triquetrous and the upper part flattened 
in one plane. Each species includes a variety with more slender aerial 
shoots which is by many authorities regarded as a distinct species; 
P. Capillare , Bl., is the variety of P. triquetrum , and P. complanatum , Sw., 
the variety of P. flaccidum. In the case of the latter P. complanatum is 
sometimes regarded as the type and P. flaccidum as the variety. From 
Baker’s 1 description the material here described is referable apparently to 
P. flaccidum, but the distinction between this and P. complanatum does not 
seem to be very clear. 
As regards geographical distribution, according to Bertrand, 2 who 
regards P. complanatum and P. flaccidum as distinct species, the former is 
found in Jamaica, Mexico, Philippine Islands, Society Isles, and Sandwich 
Isles, while P. flaccidum occurs in Java and the Philippine Islands. The 
material on which the following account is based was, however, collected 
by Professor Stanley Gardiner in the Seychelles, on the summit of Mount 
Sebert at an elevation of 1,800 feet, and was preserved in methylated spirit. 
This material consisted of the complete parts of five aerial stems, but 
unfortunately none of the rhizome was present. Some laboratory material 
of unknown origin, and which had been preserved in formalin, was also 
available for examination, but consisted of the upper aerial branches only. 
In both this material and that from the Seychelles sporangia were 
numerous. 
1 Baker, J. G. (’87), p. 30 . 
3 Bertrand, C. E. (’83), p. 11 . 
[Annals of Botany, Vol. XXIV. No. XCIV. April, 1910.] 
