SIPHONALES 89 



develop additional rhizoids that grow down and form a sheathing 

 pseudo-cortex. The bi- or tripinnate fronds usually have the 

 branching confined to one plane, the branches being constricted at 

 the point of origin, whilst the cell membrane is also thickened at 

 such places. The cytoplasm in the main axis and branches fre- 

 quently exhibits streaming movements. The function of the 

 rhizome, especially in warmer waters, is probably that of a peren- 

 nating organ, although vegetative multiplication can also occur 

 through abstriction of the pinnae, which then develop rhizoids at 

 their lower end. The only other known method of reproduction is 

 sexual. The plants are dioecious and produce anisogametes which 

 develop in gametangia that are cut off from the parent thallus by 

 means of septa. Both types of gamete are bifiagellate, but the 

 microgametes differ from the macrogametes in that they lack 

 pyrenoids. The gametes are liberated through gelatinization of the 

 apex of the gametangium, and after fusion has taken place the 

 zygote germinates at once into a new plant. The plants are diploid 

 because meiosis takes place at gamete formation ; there is therefore 

 no haploid generation. The plants can behave like Vaucheria (cf. 

 p. 95) in their response to certain environmental conditions; thus, 

 gamete formation is hastened by transference of the plants from 

 light to dark or by changing the concentration of the nutrient 

 solution. Inversion of the thallus takes place under conditions of 

 dull light or when it is planted upside down, and under these 

 circumstances the apices of the pinnae develop rhizoids. This 

 exhibition of polarity indicates clearly that the thallus is differ- 

 entiated internally, but it is still a matter for speculation as to how 

 such differentiation can occur in an organism which is to all intents 

 and purposes one unit. 



*Caulerpaceae : Caulerpa (caul, stem; erpa, creep). Fig. 62. 



Most of the species frequent the quiet shallow waters of the 

 tropics where they are often rooted in sand or mud, but two have 

 migrated far enough north to become denizens of the Mediter- 

 ranean. The prostrate rhizome is attached by means of colourless 

 rhizoids and gives rise to numerous erect, upright, assimilatory 

 shoots with apical growth, the form and arrangement of which may 

 vary very considerably (fig. 62 A-F). Radial branching is regarded 

 as primitive, whilst the more evolved forms of quieter waters 



