262 



ALG^ 



coalescence of the contents of two cells. Several species of Mesocarpus 

 frequently put out long connecting tubes between two filaments, which 

 assume a barrel-shaped form, but without any formation of zygosperms. 

 In this condition they closely resemble Mougeotia (de By.). 



The most important point in which the Mesocarpaceae differ from 

 the Zygnemacese is in the processes which take place after the formation 

 of the true zygosperm. Immediately after its formation, it divides into 

 two, three, or more cells, the central one only of which is fertile, ger- 

 minating after a period of rest ; the other sterile cells, which are separated 

 from the fertile cell by septa, taking no part in the germination. The 

 germinating cell is therefore here a resting-spore or hypnospore, produced 

 non-sexually, and the whole structure is, as Pringsheim points out, a 

 rudimentary sporocarp, indicating an approach to the higher classes of 

 Algae ; while the family is, on the other hand, con- 

 nected with the Zygnemacese through the species of 

 Zygnema in which the zygosperm is formed in the 

 connecting tube; and the best writers are by no means 

 agreed as to the limits of the two orders. 



Wittrock has described the formation of the 'spo- 

 rocarp' as taking place in three different ways in the 

 Mesocarpacese, viz.: — (i) By the tri partition of the 

 zygosperm into a hypnospore and two sterile cells; 

 when the conjugation is lateral, the sterile cells are 

 not separated from one another by the hypnospore, 

 but are permanently united with one another. (2) 

 By quadripartition of the zygosperm ; this has been 

 observed only in the case of scalariform conjugation, 

 the sterile cells being arranged two on one side and 

 one on the other side of the hypnospore. (3) The zygosperm is cruci- 

 form or H -shaped, and the sporocarp is formed from it by quinqueparti- 

 tion, two sterile cells bounding the h)-pnospore on each side; this 

 mode also only takes place in scalariform conjugation. Although these 

 characters have been used by Braun and others for the separation of 

 the genera of Mesocarpacese, Wittrock regards them as of very little 

 systematic value, since in one species, Mougeotia calcarea (Wittr.), he 

 observed all three modes of reproduction on one and the same fila- 

 ment. In this same species AVittrock also records the formation of 

 parfhenosperms, precisely resembling the normal zygosperms, but not 

 resulting from any act of conjugation. An outgro^rth springs from 

 a filament, but is not met by any corresponding outgrowth from an- 

 other filament. It is, however, cut off by a septum, and divides into 

 sterile cells and a hypnospore, just as if fecundation had taken place. 



Fig. 235. — Mesocarpus 

 pteitrocarpus de 6y. ; 

 Jateral conjugation 

 (x 200). (From na- 

 ture.) 



