CONCLUSION 295 



We must now return to the Chlorophycese. We 

 have in Ulothrix a form scarcely more complex than 

 Spirogyra, but evidently on quite a different line of 

 descent. Here the reproductive cells are all ciliated 

 and active. It is between certain of these ciliated 

 zoospores that conjugation takes place, and not between 

 vegetative cells, as in Spirogyra. Evidently the origin 

 of sexuality was quite distinct in these two groups, for 

 in UlotJirix we find its first stages, the sexual cells being 

 still capable of germinating like ordinary zoospores, if 

 unable to conjugate. If we had been able to take a 

 wider survey of the vegetable kingdom, we should have 

 found evidence that this important step from asexual to 

 sexual reproduction was made independently in many 

 groups. 



Ulothrix is peculiarly interesting, because when con- 

 jugation is accomplished it has a marked effect on the 

 future development. The solitary zoospore merely repro- 

 duces the ordinary plant, whereas the zygospore gives 

 rise to a dwarf individual quite distinct from the typical 

 form. In fact we have here, coinciding with the first 

 appearance of sexuality, the first indication of regularly 

 alternating generations. For this reason we regard 

 Ulothrix as lying more or less in the direct line of 

 descent of the archegoniate plants, in which regular 

 alternation of generations is so strikingly a character. 

 Of course this cannot be taken literally. No form now 

 living can possibly be in the direct line of descent of 

 any other form, any more than a man's cousin can be 

 his ancestor ! One cousin, however, may more than 

 another inherit the characteristics of some remote ancestor, 

 and this is all we mean in speaking of lines of descent 

 among plants. Ulothrix enables us to form an idea 



