IN LYCOPODS 39 
the soil, and capable of complete self-nourishment for an extended 
period before the production of spores. Moreover, these are produced, 
not in a single sac, as in the Moss, but in very numerous distinct 
sacs--the sporangia. These essential differences of the sporophyte are 
those which clearly define the Bryophytes from the Pteridophytes. In 
the latter the mature sporophyte is always a free-growing organism, and 
a considerable vegetative period usually precedes the formation of the 
spores. 
Referring back to our observations on the Male Fern in the previous 
chapter, it will be seen that these remarks apply there also. The most 
obvious difference between a Lycopod and a Fern is in the size of the 
leaf ; but they correspond in all essentials, 
and both show a very marked advance 
of complexity of the sporophyte over the 
Bryophyte sporogonium. On the other 
hand, the prothallus of the Fern is a 
smaller and simpler thing than that of 
L. cernuum, and stands thus in still 
stronger antithesis to the leafy plant of 
the Moss. Putting all these points 
together, it is plain that in the Pterido- 
phytes the balance in size of the 
generations is inverted as compared with 
that in the higher Bryophytes. 
In all the Bryophytes, and also in 
many Pteridophytes, the spores are all 
alike, and of small size, as we have seen 
them to be in Wephrodium and Lyco- 
podium: this is described as the BiG 23: 
“homosporous ” condition, and it may Microsporangium of Sedaginella apus in 
be accepted we a. primitive state, Bur foe "Se seen: Mee ie Mis 
in certain other Pteridophytes, and in 
all Seed-Plants, there are two different types of spore:—the relatively 
small spore, which is easily transferred when shed, and produces a small 
male prothallus ; and the large spore which, though less easily transferred, 
develops within it what is so important to the progeny—a bulky female 
prothallus stored with nutriment. This store is derived from the parent 
plant, and is thus ready to supply the young immediately after fertilisation. 
The “heterosporous” condition brings a clear advantage, notwithstanding 
that the separation of the sexes on different prothalli increases the obstacles 
in the way of fertilisation being carried out. In certain cases the two 
types of sporangia and spores start their development alike, and only 
differentiate in the later stages; for this reason, as well as on grounds 
of general comparison, the heterosporous state may be accepted as the 
later and derivative. From the example of Se/aginel/a it will be seen 
