VAUCHERIA. 109 



Growth takes place, not in all parts of the thread, but is localized at the ends 

 of the thread and its branches. This represents a distinct advance on such 

 a plant as spirog3'ra. Again, only specialized parts of the plant in vaucheria 

 form the sexual organs. These are short branches. Farther there is a great 

 difference in the size of the two organs, and especially in the size of the 

 gametes, the supplying gametes (spermatozoids) being very minute, 

 while the receptive gamete is large and contains all the nutriment for the 

 fertilized egg. In spirogyra, on the other hand, there is usually no differ- 

 ence in size of the gametes, as we have seen, and each contributes equally in 

 the matter of nutriment for the fertilized egg. Vaucheria, therefore, rep- 

 resents a distinct advance, not only in the vegetative condition of the plant, 

 but in the specialization of the sexual organs. Vaucheria, with other related 

 algse, belongs to a group known as the Siphonece, so called because the plants 

 are tube-like or sip}ion-\ike.. 



