REPRODUCTION. 605 



Chytridieas ; probably many Mucorini ; a few Peronosporeae (probably 

 Phytophthora infestans and Pythium intermedium] ; some Ascomycetes 

 and Uredineae ; Basidiomycetes. 



In the plants just enumerated, not only are spores not 

 formed sexually, but the existence of sexual organs is 

 unknown; these plants, so far as we know, are entirely 

 asexual and are reproduced only either vegetatively or by 

 means of asexually produced spores. In many other cases, 

 which we shall subsequently discuss, we meet with an absence 

 of sexuality which is less complete; in these, namely, organs 

 which are morphologically sexual organs make their appear- 

 ance, but instead of producing sexual reproductive cells, they 

 produce cells which are capable each by itself of giving rise 

 to a new individual ; in a word, they are physiologically 

 sporangia producing spores. This substitution of an asexual 

 for a sexual production of spores, is another form of apogamy, 

 and is distinguished as parthenogenesis. 



The most satisfactory method of arriving at a compre- 

 hension of the sexuality of plants is to study, on the one 

 hand, the development, and, on the other, the degeneration of 

 sexuality, and this method we will now pursue. We will, in 

 the first place, trace the development of sexuality in the 

 plants containing chlorophyll, beginning with the simplest 

 Algae, and then we will consider the degeneration of sexuality 

 with special reference to the Fungi. 



It has been already mentioned that the protophytic Algae 

 only produce spores asexually, but in some forms a diffe- 

 rentiation of these spores can be detected. In Haematococcus, 

 for instance (see Lect. I, p. i), zoospores are produced, but 

 the zoospores are not all precisely alike. In some cases 

 the protoplasm of the cell divides only once or twice, the 

 result being the formation of two or four relatively large 

 zoospores, called macrozoospores ; in others the protoplasm 

 divides a greater number of times so that a considerable 

 number of relatively small zoospores, called microzoospores, 

 are produced. Functionally these zoospores are all alike; 

 they all come to rest, and each constitutes a new Hsemato- 

 coccus. 



