THE GAMETOPHYTE GENERATION OF THE PSILOTACE^E. 
95 
with small fragments of rhizomes, and these at first, mainly on account of their 
colour, were mistaken for prothalli. The latter structures have the same character- 
istic light-brown colour of the rhizomes, but the vascular tissues give the rhizomes 
a rigidity which one soon learns to detect, and which is not a feature of the 
prothallus. In none of the specimens examined was there any trace of the pigment 
chlorophyll. One is justified, therefore, in the conclusion that, like Botrycliium* 
Lycopodium , and other subterranean pteridophyte prothalli, the gametophyte 
generation of Tmesipteris is saprophytic in its method of nutrition. 
From the numerous additional specimens discovered since the preliminary 
account was published, I have been enabled to obtain a more accurate knowledge of 
the shape and size of the prothalli. Of the first lot found all were quite small, the 
largest specimen measuring but one-eighth of an inch in length. In the material 
found later much larger specimens were obtained, several of them nearly half an inch, 
and one or two three-quarters of an inch long. So that in size alone there seems 
to be a wide range — this of course being in mature plants bearing both antheridia 
and archegonia. All of these specimens examined were cylindrical in form, but not 
necessarily straight. They became bent, curved, and sometimes twisted in curious 
forms, as if their general configuration were determined by the surrounding particles 
of rock and other obstructions met with in the soil in the course of their develop- 
ment. In one or two examples the prothallus consisted of a short attenuating un- 
branched cylindrical structure, but in the majority of the specimens examined there 
was undoubted branching, each branch terminating in a merismatic organic apex. 
In the preliminary paper t I have already figured the method of branching. In 
fig. 1 of the present paper is shown an entire prothallus. As it still shows a distinct 
and active meristem at the apex, it is evidently not fully grown. The irregular 
configuration of the cylindrical body is quite characteristic. In preparing this 
specimen most of the rhizoids were removed with the clinging particles of soil ; but 
sufficient remain to show how very numerous and how long these structures are, and 
what an immense absorbing surface they afford. The development of these rhizoids 
may be followed from such a specimen as this. At the merismatic apex one may 
observe all stages in their differentiation from superficial cells (fig. l). This specimen 
also shows developing archegonia and antheridia near the apex. 
The appearance of the apex of an older branch is shown in fig. 2. Here it will be 
noted that the merismatic nature of the cells at the apex is not so evident. The 
great length of the mature rhizoids is very characteristic. One antheridium and 
eleven archegonia are to be observed in a small surface area. The majority of the 
archegonia shown in the figure are drawn from the view looking down into the neck 
cells. One of them, however, is seen from the side view, and shows how the necks of 
the young archegonia project like tubes for a short distance beyond the surface of the 
* Jeffrey, E. C., 1898 ; Brtjchmann, 1898-1904 ; Lang, W. H., 1901. 
t Lawson, A. A. (1916), l.c. 
