57 
Farmer. — On Isoetes lacustris , L. 
eventually becomes filled with cells which are always distinct 
from those of the upper portion, both on account of their size 
and their contents, and moreover they never form archegonia. 
The prothallium thus formed lies within the spore, but not 
connected with it, as its outer cells have their own external 
wall distinct from the endospore. 
The formation of the prothallium in Isoetes presents certain 
striking features of resemblance with that of Selaginella as 
described by Pfeffer 1 , and may perhaps help to explain some 
of the peculiarities which render the oophyte of the latter 
plant so remarkable. Pfeffer states, and I am able to confirm 
his results in their chief points, that the spore is first divided by 
a wall (diaphragm) into an upper portion in which cell- division 
goes on rapidly, and a lower and much larger portion in which 
cell-formation is long retarded, and of which the protoplasm 
contains large quantities of food-material. The ‘ Prothallium ’ 
(of Pfeffer) makes its appearance before the spore has reached 
its full size, and is only followed later, after sowing the spores, 
by a free cell-formation in the infradiaphragmatic portion. 
Cell-division is brought to a close here apparently through 
the gradually increasing inability to complete the process, the 
lower part, as I have sometimes seen in 5. Braziliensis , still 
remaining undivided into cells when an embryo was already 
growing in the spore. Pfeffer has endeavoured to make a 
morphological distinction between the two kinds of tissue 
thus described, regarding the meniscus of small-celled tissue 
as the prothallium, and comparing the lower and looser mass 
to the endosperm of Angiosperms. I venture to think, how- 
ever, that such a position is untenable, and that the facts are 
to be better explained in another way, especially when Isoetes 
and Selaginella are compared in respect of their sexual 
generation. In both there is a clearly marked upper portion 
in which cell-division proceeds with great rapidity; and a 
larger basal portion in which cell-formation takes place slowly. 
That this production of cells proceeds centripetally in Isoetes 
1 Pfeffer, Die Entw. d. Keimes d. Gattung Selaginella , Hanstein’s Bot. Abhandl. 
Bd. I. 
