356 



STRUCTURE AND LIFE HISTORIES 



tioned), and to all external appearances entirely alike, 

 may produce individuals of different sex- value, some being 

 male and some female.-^ Such was also seen to be the 

 case with the marine alga Dictyota,^ the externally similar 

 male and female gametophytes being produced by spores 

 that are alike in size and other external features. 



As a further advance spores that appear to be morpho- 

 logically alike may produce plants morphologically as well 

 as physiologically different {Anthoceros, some molds). 



2. Structural. In the little club-mosses {Selaginella) 

 we found the spores unlike, not only in function but in 

 structure, those producing males being smaller than those 

 producing females — the condition of heterospory. 



Fig. 264. — Vaucheria terrestris. anth, antheridium (empty); o, oogonia. 



323. Differentiation of Sex-organs. — In many lower 

 plants there is no recognizable structural difference be- 

 tween the organs that produce the heterogametes, but a 

 step in advance in this direction is found in such a plant 

 as Vaucheria (Fig. 264), where the antheridia are struc- 



1 See, also, pp. 246 and 248. 



2 See Chapter XVIII. 



