220 



THALLOPHYTES, 



The CoNjUGAT-s: \ a family of Algae rich in genera and species, are distinguished by 

 the occurrence of reproduction by Zygospores, in addition to the simple multiplication of 

 cells by division ; swarm-spores are not formed. In one section, comprising the Meso- 

 carpeae and the Zygnemeae, the cells remain united, and form long unbranched threads, 

 the cells of which are cylindrical, and only occasionally, w^here they are in contact with 

 a firm surface, produce lateral root-like ramifications, as organs of attachment. In 

 the Desmidieae the mature cells consist usually of two symmetrical halves often separated 

 by a constriction ; the division takes place in this constriction or, at all events, symme- 

 trically, each half becoming completely developed into a perfect cell. The external 

 contour of these cells is very various ; and since their divisions always take place 

 parallel and in the same plane, as in the previous group, they form, when the cells are 

 attached to one another, filiform rows ; but very commonly they split up and live singly. 

 A comparison of the unicellular Desmidieae with those which possess a filiform arrange- 

 ment and with the Zygnemeae, shows clearly that it is a matter of subordinate importance 

 whether cells live singly or united, so long as they are similar to one another. Each single 

 cell of Spirogyra, like an isolated cell of Closterium, &c., constitutes an individual. 



The cells of the Conjugatae are distinguished by the most various configuration and 

 the most beautiful arrangement of their masses of chlorophyll ; it occurs in parietal spiral 

 bands (Spirogyra), axile plates (Mesocarpus), a pair of radiate bodies (Zygnema), or plates 

 arranged into stars (Closterium), &c. The zygospores are always resting-cells, gerrriinating 

 only after a long period of rest, even not till the next year. The formation of the zygo- 

 spores results, in the Zygnemeae, from a strong contraction of the protoplasmic sub- 

 stance in the manner shown in Fig. 6 (p. lo), although with some modifications in the 

 diff'erent genera. In the Mesocarpeae the conjugating protuberances unite in a similar 

 manner, but the zygospore is formed by the accumulated contents of the canal 

 becoming separated on both sides from the mother-cells by division-walls ; and the 

 central piece of the conjugating apparatus thus individualised is the zygospore'^. In 

 the Desmidieae the zygospore is produced in the same manner as in the Zygnemeae; 

 it developes either one, two, or four new cells, each of which splits up into two equal 

 daughter-cells capable of division. 



FIG. 156.— Germination o{ Spirogyra jugalis (after Pringsheim, Flora, 1852, no. 30) ; / a resting zygospore ; // commencement 

 of its germination ; /// the young plant further developed from a zygospore, which had been enclosed in the cell C, the 

 conjugating apparatus being still visible ; e outer cell-vvfall of the spore ;y yellowish brown layer of the cell-wall ; g the third and 

 innermost layer of the cell-wall of the spore, which forms the germinating filament ; w iu' the first septa of the germinating 

 filament, the posterior end d growing into a narrow appendage. 



The genus Spirogyra (as an example of the Zygnemece) has already been described 

 and figured several times in sect. 3 of Book I; the additional Fig. 156 will suffice, 



^ A. de Bary, Untersuchungen iiber die Familie der Conjugaten. Leipzig 1858. 



^ [Two kinds of conjugation are distinguished ; transverse, in which the cells belong to different 

 filaments ; longitudinal, in which they are parts of the same filament. See also Hassall, /. c. ; Witt- 

 rock on Mesocarpea:; Quart. Journ. Micr. Sc. 1873, p. 123.— Ed.] 



