610 
original. 
D. T. MACDOUGAL 
Now the germination and growth of these asexually pro- 
duced spores could proceed in the absence of free water, and in 
ordinary soil in which all of the water present was represented by 
Fic. 2.—The gametophyte, or sexual 
Reproduction is 
accomplished by the movement of a 
sperm (a) from the antheridium A to 
the archegonium B where it fuses with 
the egg, accomplishing fertilization. The 
sperm swims through a thin film of water 
generation of a fern. 
which may be present. The absence of 
the film by aridity is unfavorable to the 
reproduction and continuation of this 
type of vegetation. The germination of 
the egg produces the sporophyte or fern 
plant ordinarily known (see Fig. 3). 
the hygroscopic layer coating the 
minute particles of which it is com- 
posed. 
Even with this development, 
however, plants could not get very 
far from the water, since this ele- 
meat in a free state was still neces- 
sary for the activities of the gameto- 
phyte, or sexual generation. The 
sporophyte, however, continued to 
increase in size and to wax in 
importance in the life-cycle of the 
species, until finally its body was 
much larger than that of the 
gametophyte. This feature is well 
illustrated by the tree ferns in which 
the sporophyte is a massive plant 
while the prothallium, or sexual 
generation, is a small thallose 
structure only a few millimeters 
across. 
Eventually, however, the spores. 
formed by the sporophyte, capable 
of living on dry land, were germi- 
nated in place, giving rise to sexual 
individuals, which were also held 
and nursed in the tissues of the 
sporophyte. ‘Then in completion 
of the movement, accessory struc- 
tures, including the pollen-tube, 
were formed, by which the sexual 
reproductive elements might be brought together independently of 
external conditions. By these steps the seed-plant originated and 
vegetation became truly and wholly able to occupy the land—a most 
