146 



MOKPHOLOG Y. 



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 spirogyra. 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 

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

 are tube-like or siphon-\\\&. 



308. Botrydium granulatum. An example of orte of the simpler 



members of the Siphoneae is 

 Botrydium granulatum. It is 

 found sometimes in abundance 

 on wet ground which is colored 

 green or red by its presence, 

 according to the stage of de- 

 velopment. The plant body is 

 long pear-shaped, the smaller 

 end attached to the ground by 

 slender branched rhizoids (Fig. 

 143). The protoplasm contains 

 many nuclei and lines the inside 

 of the wall. When multiplication 

 takes place large numbers of 

 small zoospores with one cilium 

 each are formed in the proto- 

 plasm, and escape at free end. 

 Reproduction takes place by 

 two-ciliated gametes, which fuse 

 in pairs to form zygospores. In 

 dry seasons the protoplasm in 

 the pear-shaped plant passes 

 down into the rhizoids and 

 forms small rounded planospores. 

 All the stages of development are too complicated to describe here. 



Fig. i 43 a. 



Botrydium granulatum. A, the whole 

 plant; B, swarm spore; C, planogametes ; a, 

 a single gamete; b-e, two gametes in process 

 of fusion; /, zygote. 



