40 ALTERNATING GENERATIONS 
that heterospory may occur without any essential change in the 
sporophyte ; for the plant of Sedagznedla is of the general Lycopod type, 
with small-leaved, much-branched shoot rooted in the soil, showing. 
Fic. 24. 
&, megasporangium of Se/aginella apus in median vertical section, showing three of the 
four megaspores. X21. A, a single megaspore, with prothallus and an archegonium, 
more highly magnified. (After Miss Lyon.) 
continued apical growth and terminal strobilii These are constructed 
essentially upon the Jycopodinous plan, but instead of the sporangia. 
being all alike, some contain numerous small microspores, others contain. 
Fic. 25. Fic. 26. 
Microspore of Se/aginella apus, after Microspore of Sedaginella apus, just before extru- 
germination. (After Miss Lyon.) sion of the spermatozoids. (After Miss Lyon.) 
only four large megaspores. In both cases these result, like other spores, 
from a tetrad division: the chief difference is in their size (Figs. 23, 24 8). 
But though the sporophyte is essentially unaltered, the changes in the 
gametophyte which accompany the heterosporous state are important. The 
prothallus is no longer a free-growing, self-nourishing organism, but it 
