174 VAUCHERIA AND CAULERPA less. 



flagellate. In other words, we have a more obvious case of 

 sexual differentiation than was found to occur in Vorticella, 

 (p. 132): the large inactive egg-cell which furnishes by far 

 the greater portion of the material of the oosperm is the 

 female gamete ; the small active sperm-cell, the function of 

 which is probably (see Lesson XXIII) to furnish additional 

 nuclear material, is the male gamete. 



Similarly the oosperm is evidently a zygote, but a zygote 

 formed by the union of the highly differentiated gametes, 

 ovum and sperm, just as a zygospore (p. 166) is one formed 

 by the union of equal sized gametes. 



As we shall see, this form of conjugation — often distin- 

 guished as fertilization — occurs in a large proportion of 

 flowerless plants, such as mosses and ferns (Lessons XXX. 

 and XXXI.), as well as in all animals but the very lowest. 

 From lowly water-weeds up to ferns and club mosses, and 

 from sponges and polypes up to man, the process of sexual 

 reproduction is essentially the same, consisting in the conju- 

 gation of a microgamete or sperm with a megagamete or 

 ovum ; a zygote, the oosperm or unicellular embryo, being 

 produced, which afterwards develops into an independent 

 plant or animal of the new generation. It is a truly remark- 

 able circumstance that what we may consider as the highest 

 form of the sexual process should make its appearance so 

 low down in the scale of life. 



The nutrition of Vaucheria is purely holophytic ; its food 

 consists of a watery solution of mineral salts and of carbon 

 dioxide, the latter being split up, by the action of the chro- 

 matophores, into carbon and oxygen. 



Mucor and Vaucheria are examples of non-cellular plants 

 which attain some complexity by elongation and branching. 



