3 68 



BOTANY OF THE LIVING PLANT 



and presumably diploid swarmer. Hence, in addition to the regular 

 features of the life-cycle, certain irregularities may occur, as in other 

 classes of plants. All species with dwarf males are dioecious, i.e. the 

 male and female gametes are produced on different plants. 



A point of interest in these, as it is also in other Confervoid Algae, 

 is that they illustrate degrees of sexual differentiation. Ulothrix has 

 isogametes : Oedogonium heterogametes. It is not suggested, however, 

 that these particular genera are closely related to one another by 

 descent. Another feature is the similarity of the gametes to the 

 zoospores, of which they appear as smaller examples. This suggests 

 that gametes may actually be in origin of the nature of zoospores 

 specialised in relation to sexual fusion. 



Siphonales. 



The Siphonales have already been discussed in Chapter X., in re- 

 spect of their peculiar structure (p. 171, Fig. 105). They are coenocytes 

 that is, they are not partitioned into separate cells, but the plant-body 

 consists of a large non-septate sac, limited by a cell-wall, and kept 

 firm enough by internal turgor to preserve its form in the quiet water 

 in which as a rule these plants live. This condition is shown in a less 

 complete form in some members of the Order, where septa occur at in- 

 tervals, but the protoplasts lying between these are multi-nucleate, and 

 hardly warrant their designation as cells. This is also seen in Clado- 

 phora, a very common genus of fresh- and salt-water Algae. Clearly the 

 non-septate state is mechanically ineffective. It has been pointed 

 out (p. 172) how, as a set off against it, the more complicated forms 

 of the Siphonales have acquired additional mechanical strength, 

 either by internal cellulose ties, as in Caulerpa (Fig. 273), or by matting 



Fig. 273. 



«>?"? ? f a transverse section of Caulerpa, showing the thick outer wall, and the 

 reticulate rods of cellulose, which act as ties, and give added rigidity. F. O. B. 

 I x 5°-) 



