374 
Stiles .— The Structure of the Aerial Shoots of 
To Professor Seward my thanks are due for handing over the material 
to me for examination. I would also thank him and Mr. Tansley for their 
advice given with regard to the work and for their interest in its progress. 
To Mr. H. H. Thomas I am indebted for advice and help with the 
photomicrographs. 
External Morphology. 
The sporophyte of Psilotum flaccidum consists of a branched rhizome, 
certain branches of which grow out towards the surface of the substratum, 
and ultimately become aerial stems. As in all the Psilotaceae, there are no 
roots. The plant is epiphytic and of pendulous habit; its rhizome appa¬ 
rently grows in the humus attached to trees, from which its aerial branches 
hang down in clusters. 1 
Our knowledge of the rhizome is practically limited to the references 
to it in the monographs of Bertrand, 2 Solms-Laubach, 3 and Pritzel, 4 from 
which it would appear to be very similar to that of P. triquetrum. The 
branches of the rhizome which are destined to become aerial shoots pass 
through a transition region before reaching the surface of the substratum. 
All my five specimens of aerial shoots are from 15 to 20 centimetres 
long, and in each case at its very base the aerial stem is cylindrical and of 
a dark colour. Higher up this stem loses its dark colour and becomes 
wider, and soon passes over into a triquetrous region, which persists for 
a shorter or longer distance, and itself gradually changes into the flattened 
form characteristic of the greater part of the aerial branches (PI. XXV, Fig. 1). 
The triangular lower part and flattened upper part are clearly shown in the 
early figure of P. complanatum published by Swartz 5 in 1806. 
In Psilotum triquetrum the branching is apparently dichotomous, the 
planes of successive dichotomies being at right angles. This would seem 
to be the case in P. flaccidum so long as the stem is triangular 6 (Fig. 1), 
but where the stem is flattened the branching takes place all in one 
plane. 
The stem bears two kinds of appendages, leaves and sporophylls. The 
former are small scale-like structures about a millimetre long and lanceolate 
in shape. The sporophylls are forked, the lower part below the two prongs 
being extremely short, so that the sporophyll has much the appearance of 
two leaves closely connate at their base. Between the two lobes of the 
sporophyll is the sporangiophore. This consists of a very short axis 
bearing at its apex the three confluent sporangia. Bilocular synangia are 
also by no means uncommon. 7 
1 Pritzel, E. (’00), p. 610 . 2 Bertrand, C. E. (’83). 3 Solms-Laubach (’84). 
4 Pritzel, E., 1. c. 5 Swartz, O. (1806), pp. 188 , 414 , t. 4 , Fig. 5 . 
£ Branching in the triquetrous region of the stem is rare, and my specimens were few in number, 
so that this point could not be definitely ascertained. 
7 Cf. Psilotum triquetrum , Miss Ford (’04), p. 591 . 
