260 ELEMENTS OF BOTANY 
Phanerogams are distinguished from all other plants by 
their power of producing seeds, or enclosed macrosporangia, 
with embryos. 
337. The Law of Biogenesis and the Relationships of the 
Great Groups of Plants. — On summing up Sects. 334-336, 
Wy /| 
Fic. 178. Longitudinal Section 
through Fertilized Ovule of a 
Spruce. 
p, pollen grains; ¢, pollen tubes; 
n, neck of the archegonium; 
a, body of archegonium with 
nucleus; e, embryo sac filled 
with endosperm. 
it is evident that the sexual 
generation in general occupies 
a less and less important share 
in the life of the plant as one 
goes higher in the scale of 
plant life.t In the case of the 
rockweed, for instance, the 
sexual generation is the plant. 
Among mosses the sexual gen- 
eration is still very prominent 
in the life of the plant. Ordi- 
nary ferns show_us the sexual 
generation existing only as a 
tiny independent organism, 
living on food materials which 
it derives from the earth and 
air. In the Salvinia it is re- 
duced to microscopic size and 
is wholly dependent on the 
parent plant for support. 
Among seed-plants the sexual 
generation is so short-lived, 
so microscopic, and so largely 
enclosed by the tissues of the flower that it is compara- 
tively hard to demonstrate that it exists. 
1A good many plants of low organization, however, are not known to pass 
through any sexual stage. 
