Conjugate 329 



In all members of the Conjugate there is a firm wall of cellulose and in 

 addition all these Algae are remarkable for the great development of the 

 mucilaginous pectose constituents of the cell-wall. There is often either a 

 conversion of the outer cellulose layers into mucilage or a continuous 

 exudation of mucilage, until, in many instances, the gelatinous envelope is 

 of much greater bulk than the individual plant. The unicellular forms 

 not infrequently occur embedded in a mass of transparent jelly formed by 

 the coalescence of their outer mucilaginous coverings. All Conjugates, 

 with the possible exception of Sirogonium sticticum, are slimy to the 

 touch. 



One of the most conspicuous features of the Conjugate is the large size and 

 definite form of the chloroplasts. There are from one to about eight or twelve 

 in each cell and they exhibit great variety in form and disposition. Each 

 chloroplast contains one or more pyrenoids, which are often very conspicuous 

 and frequently arranged in a symmetrical manner. Starch is the principal 

 food-reserve. The nucleus is in nearly all cases central in position. 



In a number of widely scattered types of the Conjugates the cell-sap is 

 coloured purple or violet by a soluble pigment first investigated by Lagerheim 

 ('95) and named by him phycoporphyrin. Among Conjugates in which this 

 pigment is present may be mentioned Mesott&nium violascens De Bary, 

 M. purpureum W. & G. S. West, Mougeotia capucina (Bory) Ag. and Plearo- 

 discus purpureus ( Wolle) Lagerh. 



Multiplication takes place by cell-division, which in the Desmidiacese is 

 the usual method of propagation. Even in the filamentous Zygnemacese the 

 filaments often break up into single cells or short chains of cells, which by 

 further divisions quickly form new filaments. In many species of Spirogyra 

 this fragmentation is facilitated by a special mechanism in the transverse 

 walls (vide p. 349). 



Asexual reproduction sometimes occurs by the formation of aplanospores, 

 both in the Zygnemaceas and the Desmidiaceoe, but is for the most part 

 rather unusual. Resting ' cysts,' consisting of one or several cells with thick 

 walls, are often formed in Zygnema, 



Sexual reproduction of a low type occurs by the conjugation of non-ciliated 

 isogametes of relatively large size. The ordinary vegetative cell becomes a 

 gametangium, usually without change of form, and gives origin only to one 

 gamete. The latter is derived from the whole contents of the gametangium 

 except in Mougeotia and Pyxispora, in which some of the protoplast is left 

 unused within the gametangium. In the Zygnemacerc the gametes unite 

 anisogamously within one of the gametangia or isogamously within the 

 conjugation-tube which joins the two gametangia, but in the Desmidiaceae 

 (with one solitary exception) the zygote is formed isogamously between the 

 two empty gametangia. The zygospore is in all cases a resting spore. 



