164 



STRUCTURAL BOTANY 



a larger (Fig. 70, C), for of course the size of the swarmers 

 varies according to the number produced in a mother-cell, 

 and the dimensions of the latter. 



Apart from such accidental differences, we have here a 

 union of perfectly similar cells, whereas in all the plants 

 described above there was a sharp distinction between the 



male and female cells. 

 We have in fact reach- 

 ed in Ulothrix the 

 lowest and simplest 

 stage of sexual repro- 

 duction in plants. The 

 process in this rudi- 

 mentary form requires 

 a special name ; fer- 

 tilisation is the union 

 of unlike cells, e.g. 

 that of an ovum with 



FIG. 70. Ulothrix zonata. A, part of a 

 filament ; most cells are already empty ; 

 from one the biciliate zoospores are 

 escaping. B, zoospores ; C, two in the 

 act of conjugating ; D, two young 



or 



a spermatozoid, 

 with a generative cell 

 from the pollen-tube ; 



zygospores, immediately after conjuga- 

 tion ; E, ripe zygospore ; F, unicellular 

 plant grown from zygospore ; G, similar 

 plant producing zoospores, which are 

 about to escape. Magnified 482. (After 

 Dodel-Port.) 



the sexual fusion of 

 similar cells is termed 

 conjugation, and the 

 result of such a union 

 is called a zygospore, 



to distinguish it from the ob'spore, which is the product 

 of fertilisation. Every possible intermediate stage 

 between the two processes is, however, known in various 

 Algce, so we need not hesitate to regard conjugation as 

 really a primitive form of sexual reproduction. One 

 point of importance must be noticed : only swarm-cells 

 from different mother-cells will conjugate, not those 



