CONCLUSION 297 



We must now return to the Chlorophycere. 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 Ulothrix we find its first stages, the sexual cells 

 being still capable of germinating as neutral spores, 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 also interesting from the great difference 

 in subsequent development between the asexual and the 

 sexual swarm-cells. The asexual 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, some slight suggestion of regularly alter- 

 nating generations. For this reason Ulothrix has been 

 regarded 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, it is true, may more than an- 

 other inherit the characteristics of some remote ancestor, 

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

 among plants. In the case of Ulothrix, however, there 



