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. 



The important point is that 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 reached 

 in Ulothrix the lowest 

 and simplest stage of 

 sexual reproduction 

 in plants. The pro- 

 cess in this rudi- 

 mentary form requires 

 a special name; fer- 

 tilisation is the union 



of unlike cells, e.g. 

 A 'D G 



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 

 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.) 



that of an ovum with 

 a spermatozoid, or 

 with a generative cell 

 from the pollen-tube ; 

 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 oospore, which is the product 

 of fertilisation. Every possible intermediate stage 

 between the two processes is, however, known in various 

 Algse, 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 



