6l2 LECTURE XXII. 



through the incompletely differentiated planogametes of Ecto- 

 carpus and Cutleria to the highly differentiated gametes of 

 Fucus and the higher Algae, the Muscineae, and the Pterido- 

 phyta. In all these cases the product of the coalescence of the 

 two sexual cells, whether externally different or similar, is a 

 cell which, unlike the two cells which fuse to form it, is capable 

 by itself of developing into a new individual ; in a word, it is 

 a sexually produced spore. 



Before passing to the consideration of sexuality in the 

 Fungi, we may conveniently discuss the peculiar modes of the 

 sexual process which are characteristic of certain groups of 

 plants which possess chlorophyll, namely, the Conjugatae and 

 the Florideae among the Algae, and the Phanerogams. 



C A B 



Fig. 69 (after Strasburger). Stages in the conjugation of Spirogyra. 



The group of the Conjugatae, including the Zygnemeae, 

 Mesocarpeae, and Desmidieae, is made up of plants which are 

 either unicellular, or consist of filaments of similar cells. In 

 all these plants the sexual process is of this kind, that the 

 walls of two adjacent cells, whether they be isolated cells, or 

 whether they form part of two contiguous filaments or, 

 rarely, when they form part of the same filament (Rhynco- 

 nema) grow out towards each other into protuberances which 

 at length come into contact. The partition walls then undergo 

 absorption, and thus a channel of communication is set up 

 between the two cell-cavities (Fig. 69). The protoplasm of 

 the two communicating cells fuses to form a spore, but this 

 fusion does not take place in quite the same way in all cases. 

 In the Zygnemeae (Fig. 69), the protoplasm of one of the two 

 cells contracts before that of the other, and travels through 



