166 STKUCTURAL 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, behaving 

 exactly like the purely asexual quadriciliate zoospores. 

 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 zoospores, 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. We see, however, that conjugation, 

 when it does take place, makes a great difference ; with- 

 out it, the isolated micro-zoospores germinate directly into 

 new plants, but if union has taken place, the develop- 

 ment is totally different, resulting in a resting unicellular 



