SUMMARY AND CONCLUSION 293 
zoids within this, which are discharged, with the 
contained water, into the cavity above the archegonium, 
and fertilize the latter in the same way as among the 
Pteridophytes. 
Comparing the homologies of the higher Pteridophytes 
and the flowering plants, we find that both produce two 
sorts of sporangia, macrosporangia and microsporangia, 
known usually among the latter group as ovules and 
pollen-sacs. In the latter, spores develop precisely as 
in all the Archegoniates from the lowest to the highest, 
i.e. by the division of each sporogenous cell into four 
spores. The macrosporangium, or ovule, of the Sper- 
matophytes generally contains but a single macrospore, 
or embryo-sac, although there are some exceptions to 
this rule. Very often one or both of the preliminary 
divisions in the sporogenous cell are suppressed. The 
sporangia of the Spermatophytes are usually borne upon 
sporophylls — carpels or stamens — which are the homo- 
logues of the sporophylls of the Pteridophytes. 
Of the Spermatophytes, the Gymnosperms are obvi- 
ously the lowest types, 7.e. they show more clearly their 
derivation from the Pteridophytes. Their more primi- 
tive character is borne out both by a study of their struct- 
ure and by their geological history. It is not likely 
that all the Gymnosperms constitute a homogeneous 
class. It is much more probable that they represent 
the remnants of two, and possibly more, quite distinct 
developmental lines. The Cycads show close affinity 
with the true ferns, while the Conifers recall more 
strongly the Lycopods. Both of these groups, espe- 
cially the Cycads, are much less abundant at the present 
time than in earlier periods of the earth’s history. 
