SPORE-PRODUCING MEMBERS 317 
define the future sporogenous tissue from the sporangial wall, but that by 
further periclinal divisions of the superficial cells additions may be made 
to it. If this be so, then 
the distinction between 
the two sources of origin 
of the tapetum does not 
appear so marked as at 
first sight it might appear 
to be. 
In the microsporangia 
all the cells of the sporo- 
genous group may under- 
go the _ tetrad-division, 
and form microspores ; 
but Miss Lyon found 
that in S. apus not more 
than five-sixths of them 
were fertile, the rest dis- 
appear. In the mega- 
sporangia, as a rule, a Fic. 163. 
single cell is early differ- Selaginella spinulosa. A, B=radial sections through young 
+ : sporangia of successive ages. C=a transverse section of one more 
entiated by its denser advanced. D=a tangential section. =a radial section of an older 
protoplasm from the rest: ;onujinaporopit cpp ses geez the ligule and 
this alone undergoes the 
tetrad-division, and forms megaspores (Fig. 164). But in S. rupestris a 
smaller number of megaspores, or even only one, may come to maturity: 
in .S. apus, however, two mother-cells 
may become matured, and eight mega- 
spores be thus formed in one sporangium. 
These fluctuations have their interesting 
bearing upon the origin of the hetero- 
sporous differentiation, showing that there 
is some margin of variation in the num- 
ber of spore-mother-cells which are fertile 
even in forms now living. 
The facts relating to the sporangium 
in Selaginella, though imperfectly known 
for the genus at large, show that in 
Fic. 164. position and in general plan the spor- 
Selaginella spinulosa, A. Br. Section of angium is of the usual Lycopod type ; 
megasporangium showing the single fertile : 3 i 
tetrad still very small, and the rest of the but that its dimensions are smaller than 
Sen ee teet ee is usually the case in Lycopodium: the 
difference in origin of the tapetum is probably related to the smaller size 
of the whole sporangium. The heterosporous condition appears to have 
brought with it only minor modifications of the original sporangial type. 
