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, CEdogonium, 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 indimdiials only, of simple structure, which fasten 

 themselves to the egg-bearing plants, and give rise to 

 sperms, which fertilize the tgg.^ 



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 certain seed-bearing plants, where the 

 sporophytes are differentiated - into microsporophytes 

 (staminate plants), bearing only microspores which pro- 

 duce male gametophytes (pollen-grains), and megasporo- 

 phytes (pistillate plants), bearing only megaspores which 

 produce female gametophytes (embryo-sacs) . Usually the 

 two kinds of sporophytes are essentially alike, except for 

 the sporophyll-bearing branches (the flowers), but the 



^ It has been suggested that these androspores might be regarded as 

 sperms developing without fertilization, i.e., by parthenogenesis. 



