160 EVOLUTION OF PLANTS 
Known as a seed. This peculiar modification of the 
macrosporangium to form a seed is the real distinguish- 
ing characteristic of the Spermatophytes. 
The microspores or pollen-spores of the seed plants 
differ very little from those of the ferns either in form 
or development, and indeed are strictly homologous with 
the spores of all Archegoniates, where, as we have seen, 
the spores invariably arise from the division of the 
sporogenous cell into four equal parts. 
Owing to the position of the archegonium within the 
macrosporangium, the method of fertilization is different 
from that in the Pteridophytes, where the free gameto- 
phytes are directly exposed to the action of water, and 
motile spermatozoids are produced in the antheridium. 
The pollen-spore of the Spermatophytes on germination 
produces a long tubular filament within which is con- 
tained the very rudimentary antheridium with usually 
two sperm-cells. In its growth the pollen-tube grows 
down through the tissues above the apex of the female 
gametophyte, and finally reaches the archegonium, where 
it discharges the sperm-cells, one of which fuses with 
the egg-cell, thus effecting fecundation. Until very 
recently it was supposed that the absence of motile 
spermatozoids formed an absolute distinction between 
Pteridophytes and Spermatophytes, but the discovery of 
large fern-like spermatozoids in certain Cycads, as well 
as in the curious genus Gingko, has broken down the 
last barrier between the two groups. 
The “flower” in most Spermatophytes is a collection 
of sporophylls, or spore-bearing leaves, the carpels, 
bearing macrosporangia (ovules), and the stamens, bear- 
ing the microsporangia (pollen-sacs), These sporophylls 
