THE PROBLEM OF SEX IN PLANTS 357 
turally very unlike the oogonia. This differentiation is 
carried a step further with the appearance of the multi- 
cellular archegonium in liverworts and mosses. 
324. Structural Differentiation of the Sexes. i. Par- 
tial. In some species (though not in all) of the fresh- 
water alga, (Edogonium, the spores are unlike. Large 
zoospores produce normal-sized plants that bear eggs 
and smaller androspores. The androspores, intermediate 
in size between sperms and zoospores, produce smaller, 
male individuals only, of simple structure, which fasten 
themselves to the egg-bearing plants, and give rise to 
sperms, which fertilize the egg. 1 
2. Complete. In the liverworts and mosses commonly, 
and in the higher plants always, the gametophytes are 
clearly differentiated into male and female, with unlike 
vegetative characters which clearly distinguish them. 
These unlike structural features are called the secondary 
sexual characters. In these groups the sporophytes are 
not differentiated in structure, but the spores they pro- 
duce, though structurally alike, are physiologically unlike, 
some producing male gametophytes, others female. 
The most complete expression of maleness and female- 
ness is found in the seed-bearing plants, where the sporo- 
phytes are differentiated into microsporophytes (stami- 
nate plants), bearing only microspores which produce 
male gametophytes (pollen-grains), and megasporophytes 
(pistillate plants), bearing only megaspores which pro- 
duce female gametophytes (embryo-sacs). Usually the 
two kinds of sporophytes are essentially alike, except for 
the sporophyll-bearing branches (the flowers), but the 
1 It has been suggested that these androspores might be regarded as 
sperms developing without fertilization, i.e., by parthenogenesis. 
