I08 THE ALGAE 



(c) The septum develops an I piece and then when the wall in- 

 verts, due to increased turgor, the I piece is slipped off and 

 the two cells come apart (cf. Fig. 59, c-e). 



When two filaments touch they may form joints or genkulations, 

 adhesion being brought about by a mucilaginous secretion pro- 

 duced by the stimulation of the contact. The formation of such 

 geniculations, however, has no connexion with reproduction. 



Sexual reproduction is secured by the process of conjugation, 

 the onset of which is brought about by a combination of certain 

 internal physiological factors combined with the pH of the external 

 medium. It commonly takes place during the spring phase and then 

 the threads come together in pairs, but either one or more than two 

 filaments may also be involved. The threads first come together by 

 slow movements, the mechanism of which may be connected with 

 the secretion of mucilage; then they become glued together by 

 their mucilage and later young and recently formed cells in both 

 filaments put out papillae. These papillae meet almost immediately, 

 elongate, and push the threads apart. Normally one of the threads 

 produces male gametes and the other female, but occasionally the 

 filaments may contain mixed cells. The papillae from male cells 

 are usually longer and thinner than those from the female cells and 

 so they can fit inside the latter. The conjugating cells accumulate 

 much starch, the nuclei decrease in size and the wall separating the 

 papillae breaks, thus forming a conjugation tube. The whole pro- 

 cess so far described forms the maturation phase which is followed 

 by xho^phase of gametic union. Contractile vacuoles, which make their 

 appearance in the cytoplasmic lining, remove water from the central 

 vacuole and so cause the protoplasm of the male cells to contract 

 from the walls. The male cytoplasmic mass then migrates through 

 the conjugation tube into the female cell where fusion of the two 

 masses takes place and this is then followed by contraction of the 

 female cytoplasm, though in the larger species it may contract 

 before fusion. Fusion of the two nuclei may be delayed for some 

 time, but in any case the male chloroplasts degenerate. The process 

 described above is known as scalariform conjugation, and it in- 

 cludes certain abnormal cases where cells produce more than one 

 papilla or where the papillae are crossed. In some monoecious 

 species, however, lateral conjugation occurs, the processes being 

 put out from adjoining cells on the same filament. More recently a 

 species (5. jogensis) has been described in which contents of ad- 



