26o ALG.S 



Order i.— Mesocarpace^e. 



The species belonging to this family consist of cylindrical unbranched 

 or very rarely branched filaments of elongated cells, in which the chloro- 

 phyll is not arranged, as in the Zygnemaceae, in stars or spiral bands, 

 but in a thin axile plate occupying one diameter in each cell, and con- 

 taining a number of conspicuous starch-grains ; those of adjacent cells 

 lying usually or invariably in the same plane. Vegetative propagation 

 takes place by the breaking lip of a filament into its constituent cells ; 

 sexual reproduction by a process of conjugation, which may take place 

 either between cells of the same or of different filaments. 



The ordinary mode of conjugation in the Mesocarpacese is that 

 termed scalariform, viz. between the several cells of two different fila- 

 ments. In most species of Mesocarpus (Hass.) this takes place in the 

 following way. When two filaments lie very near one another side by 

 side, each cell of each filament puts out a short protuberance on the 

 side facing the other filament. 'While these are forming, the greater 

 part, but not the whole, of the endochrome in each cell passes into the 

 protuberance thus formed, a portion being apparently always left behind 

 As soon as the two protuberances meet, the cell-wall becomes absorbed 

 at the extremity of each, and an open tube is thus formed in which the 

 protoplasm of the two conjugating cells coalesces, with expulsion of cell- 

 sap and consequent contraction into a globular zygosperm. The zygo- 

 sperm is not formed in the centre of the short tube, but at one extremity 

 of it, in contact with what may possibly be regarded as the female fila- 

 ment, although the differentiation is doubtful, and in any case exceed- 

 ingly slight. The zygosperm may be the result of the coalescence of 

 three cells instead of two. The zygosperm is at once separated from 

 the rest of the conjugating tubes, or from the mother-cells, by a septum on 

 either side. In Staurospermum (Ktz.), where the cells are very long and 

 narrow, four cells take part in the formation of each zygosperm. Two of 

 the slender filaments, lying side by side, bend towards one another con- 

 vexly so as to bring a part of each filament where there is a septum in 

 contact. Both the septa and the longitudinal bounding-walls become 

 absorbed at this spot, and the greater part of the contents of the four 

 cells coalesce into a zygosperm, which is often of a more or less quadrate 

 form, and is again sharply marked off from the four mother-cells by septa 

 at the truncated corners. Lateral conjugation also takes place in some 

 species of Mesocarpus between two adjacent cells in the same filament. 

 In this process each of the two cells puts out a horn-like protuberance at 

 the end adjacent to the other cell ; these protuberances bend towards one- 



