FRESH-WATER ALG.E. 101 



with a solid central medullary stratum, surrounded by dichoto- 

 mously divided branchlets. The asexual reproductive organs or 

 gonidia are generally formed in fours in a mother cell ; hence they 

 are termed Tetragonidia. The sexual reproduction is usually 

 dioecious ; the antheridia are single cells at the end of long articu- 

 lated branches, each producing only one antherozoid ; the carpo- 

 gonium consists of a single cell, which is prolonged upwards into a 

 trichogyne. The roundish antherozoids have no cilia and do not 

 swarm, but are moved along passively by the water ; some of them 

 are thus brought into contact with the trichogyne, adhere to it, 

 and in consequence of the absorption of the cell walls at the 

 points of contact, their contents pass into it, the trichogyne 

 remains otherwise permanently closed. After fertilisation the 

 basal portion of the carpogonium becomes multicellular, in conse- 

 quence of divisions having taken place ; the cells thus formed 

 bulge outwards, and give rise to a dense aggregation of short 

 branches, the terminal segments of which are the carpospores. 

 This simple sporocarp acquires a loose investment by the out- 

 growth of prolongations from the cells beneath the carpogonium. 

 Lemanea probably belongs to this class ; the thallus is setaceous, 

 almost simple, hollow, nodose, having an internal and a cortical 

 layer of cells, the latter of a brownish or obscure colour. Lemanea 

 is said to flourish in rapid currents, contrary to the habits of most 

 algae — they may be found in mill sluices and in the most im- 

 petuous cascades. Chantraiisia and Bangia are both doubtful 

 members of this class ; the thallus is filamentous, and purple or 

 violet in colour, and tetraspores are found in each genus. 



As regards the process of sexual reproduction, it has as yet 

 been observed only in a small proportion of the numerous genera 

 of Algae ; nevertheless, it is permissible to assume that in some of 

 the remaining genera such a process actually takes place, an 

 assumption which is strengthened by a similarity of development 

 in certain instances between the two. 



The whole subject is, however, itself in a state of development, 

 and the present scheme will probably require alteration and re- 

 arrangement before it can be universally accepted. The presence 

 of the fructification is an important point in determining a given 

 species of Algae. Cooke says that a great many species described 



VOL. V. I 



