166 STRUCTURAL BOTANY 



the zoospores of that plant. Now in Ulothrix it is not 

 a mere matter of similar appearance ; the sexual cells 

 actually are zoospores, for if conjugation be not effected 

 they can germinate on their own account, either behaving 

 exactly like the purely asexual quadriciliate zoospores, 

 or forming resting-cells like the zygospores. 1 This 

 may happen either within the mother-cell, if they fail 

 to free themselves, or in the open, if no opportunity for 

 conjugation has arisen. The little zoospores then come 

 to rest, and grow out at once into Ulothrix plants smaller 

 than those formed from the big zoosporee, but otherwise 

 quite like them. It is true that the very smallest swarm- 

 cells (produced to the number of thirty-two or more in a 

 mother-cell) sometimes only give rise to a very feeble 

 and rudimentary thread, which may never grow up into 

 a mature plant. We see, however, this important fact : 

 the conjugating cells have not yet become exclusively 

 adapted to a sexual function ; they are still spores, 

 capable of more or less successful independent germina- 

 tion, though fitted under favourable conditions for con- 

 jugation. This makes Ulothrix and other Algae like it of 

 quite special interest, for in them we can trace the very 

 first rise of the sexual process among cells which are still, 

 to all intents and purposes, spores. 



Conjugating motile cells are often called planogametce 

 (which implies that they are wandering sexual cells), but 

 we have not used that word so far, because it is important 

 to bring home to our minds the really fundamental fact, 

 that the cells which conjugate are themselves spores. 



1 According to a recent observer, the true sexual cells, when they fail to 

 conjugate, always behave like zygospores. The small zoospores which 

 germinate directly are said not to be identical with the sexual swarm- 

 cells, though almost indistinguishable from them. 



