2l8 THE ALGAE 



dealing with an auxiliary cell in just the same sense as there are 

 haploid carpospores in such genera. Carpospores are normally 

 formed from a pecuUar diploid generation that develops para- 

 sitically on the female plant. They are produced in sporangia that 

 terminate filaments that arise usually from the auxiliary cell or 

 fertiUzed carpogoniimii. These filaments are known as the gonimo- 

 blasts. In the primitive genera, e.g. of Nemalionales, where reduc- 

 tion division occurs at germination of the zygote the carposporo- 

 phyte is haploid, but in all the others it is a parasitic diploid plant 

 growing on the parent haploid plant. On germination the diploid 

 carpospores give rise to a new independent asexual diploid genera- 

 tion, which reproduces by means of tetraspores formed in sporangia 

 borne externally or else sunk into the thallus (Figs. 141, 145). In 

 the majority of species where there are two separate diploid 

 generations the plants can be termed morphologically triphasic 

 but cytologically diphasic. 



A common feature of the group that further emphasizes their 

 uniformity is a tendency for the 2n ntimber of chromosomes to be 

 40. The Rhodophyceae may be regarded as a classical example of 

 plants in which meiosis occurs at different phases in the life cycle, 

 for it may either occur immediately on germination of the zygote 

 or at some later period. When meiosis is delayed there is either a 

 carposporophyte {Liagora tetrasporifera^ Helminthocladia) or car- 

 posporophyte and tetrasporophyte alternating with ±e sexual 



generation. 



The classification of the Rhodophyceae is based primarily upon 

 the structure of the female reproductive apparatus. After the Pro- 

 toflorideae, which mostly lack pit connexions and have a single axile 

 plastid in each cell, have been segregated, the remainder of the red 

 algae, or Euflorideae, are classified as follows : 



(i) Nemalionales: 



Construction fountain type or central filamentous. Carpogonia 

 occur singly, and the so-called auxiUary cell, if present, is 

 formed from a carpogonial branch cell or its derivative and is 

 nutritive. Reduction division normally occurs immediately after 

 fertilization but exceptions are known {Galaxaura). When 

 present the tetrasporangia are cruciate. 



(2) Gelidiales: 



Central filament type of construction. The sessile carpogonia 



