244 



PART III. THE CLASSIFICATION OF PLANTS. 





forms, the plant is the gametophyte, producing not only the sexual cells, but 

 also zoogonidia. The sporophyte is the zygospore (Ulothrix), or the oospore 

 (CEdogonium), or a structure developed by cell -division from the oospore 

 (Coleochaete). la Ulothrix and CEdogonium, the contents of the spore, whether 

 zygospore or oospore, undergo division giving rise to two or more zoospores 

 which are set free and, after a longer or shorter period of movement, come to 

 rest and germinate, each developing into a gametophyte. In Coleochaete, the 

 contents of the oospore likewise undergo division, but the product is not several 

 zoospores ; it is a small multicellular body which constitutes the sporophyte, 

 each cell of which eventually sets free its contents as a single zoospore which, 

 on coming to rest, developes into a gametophyte. 



It must be noted that in this series, as in others, the gametophyte does not 

 always actually produce sexual organs ; thus, in Coleochaete, there is generally 

 a succession of such potential gametophytes produced by means of zoogonidia 

 before an actual gametophyte makes its appearance. 



There is evidence of considerable further polymorphism in the members of 

 this group : some of them appear to have protococcoid or palmelloid forms ; 

 the flattened expanded forms have filamentous stages, etc. 



The following is a brief account of the orders of the Confervoideas : 

 Order 1. Conjugates ; the characteristics of this sub-order have been 

 already stated ; it need only be added that the plants are usually not attached. 

 It includes the families Desmidiese, Zygnemeae and Mesocarpeae ; all fresh- 

 water. 



Family 1. Desmidiece. These are unicellular organisms, either solitary or 

 connected into filaments ; they possess some power of locomotion. Each cell 

 consists of a mass of protoplasm with a central nucleus, and contains two 

 or more chloroplastids in which lie conspicuous pyrenoids. The cell-contents 

 are arranged symmetrically in the two halves of the cell, and in many forms 

 this bilateral symmetry is emphasised by a deep constriction in the median 

 plane. 



The Desmids multiply to a large extent by division ; the cell is divided into 



two by the formation of a cell-wall 

 in the median plane, and then each 

 half produces a new half correspond- 

 ing to itself ; hence the two halves 

 of a cell are of different ages. The 

 only other mode of reproduction is 

 by means of zygospores formed by 

 tbe conjugation of two individuals 

 (Fig. 173). On germination the con- 

 tents of the zygospore divide into 

 two halves, each of which becomes 

 an individual. 



Among the commoner forms of 

 the DesmidieaB are Closterium (Figs. 

 173 and 174), Staurastrum, Euas- 

 trum (Fig. 174 <7.). 

 Family 2. Zygnemea. These plants, consisting of long, delicate, unbranched 



cl 



FIG. 173. Closterium. A in conjugation; 

 zy zygospore : B in transverse section show- 

 ing position of the chloroplastid cl (x 200). 



