THE GAMETOPHYTE GENERATION OF THE PSILOTACE^E. 
103 
comparatively few in number, but on the majority of the older prothalli, as seen in 
fig. 19, the antheridia could be counted by the score. In such old prothalli the 
antheridia were so numerous that an almost complete series of developmental stages 
could be obtained from a single specimen. 
From a superficial study it becomes evident that the antheridia are not confined 
to any particular region. They may be observed on all sides, and apparently develop 
from any surface. They originate and develop with the young tissue at the growing 
point. It was only in such apical regions that the developmental stages were found. 
Only mature antheridia were observed on the older-established prothallial tissues. 
Serial sections beginning at the apex showed all the essential stages of development. 
Some of these sections are represented in figs. 23, 26, and 27. From these figures 
the evidence is quite conclusive that the antheridia are borne upon all sides, and not 
limited to any one surface. Fig. 23 represents a transverse section through the 
prothallus some considerable distance behind the apex. Four mature antheridia are 
shown. One of the latter is empty, and has evidently discharged its contents. The 
other three clearly show numerous coiled spermatozoids. In fig. 26 we have another 
section from the same prothallus. In the one plane we may see three antheridia 
alternating with four archegonia. Two of these antheridia contain coiled spermato- 
zoids, and the archegonia are evidently ready for fertilisation. Fig. 27 represents 
another section similar to that shown in fig. 26. Two antheridia are to be seen, one 
of which has discharged its gametes. 
From these figures it became quite evident that the antheridia are almost 
spherical in form and extend out quite conspicuously from the surface of the 
prothallus (figs. 23, 26, and 27). In this regard they stand in sharp contrast to 
the type of antheridium described for Equisetum, Lycopodium , Ophioglossum , 
Botrycliium, etc. 
In all these types the antheridium develops below the surface of the prothallus. 
The antheridium of Psilotum is in this regard exactly like that of Tmesipteris, as 
described above. I know of no other subterranean types that have this superficial 
antheridium. It is a point of some significance that Psilotum and Tmesipteris 
should have this feature in common. 
In regard to the development of the antheridium, a complete series of stages was 
obtained w r ith the exception of the first initial cell. It would be difficult to identify 
such a stage as this from other cells near the merismatic region. As development 
proceeds, however, one may very easily identify the young stages. Fig. 28, for 
instance, represents a median section through a young antheridium. It extends out 
very slightly from the surface of the prothallus. There are four external wall cells 
and two inner cells as seen in the section. The inner cells stand out very con- 
spicuously by their dense granular cytoplasm and very large, deeply staining nuclei. 
An examination of the sections preceding and following this one showed that the 
inner portion of the antheridium consists of two cells at this stage. A slightly older 
