REPRODUCTION AND PROPAGATION u 
Thus in the Equisetaceae the spores are all alike, but the 
prothalli unisexual; in Lycopodiaceae the spores are all alike 
and the prothalli monoecious, while in the otherwise very 
similar Selaginellaceae the spores are of two kinds — one pro- 
ducing spermatozoids and the other archegonia on germination, 
with only very rudimentary prothalli. Thus the sexual differ- 
entiation which shows only on the prothallus in ferns begins 
in the spores of Selaginella , and though the alternation of 
generations is still present, it is in a highly modified form. 
