254 PART IV. CLASSIFICATION. 



An asexual formation of spores takes place in all the Confervoideae 

 except the Conjugatse. A sexual formation of spores takes place in all 

 the Confervoidese ; in the isogamous forms the product of conjugation is a 

 zygospore ; in the oogamous forms it is an oospore. In the isogamous 

 forms the sexual organs, or gametangia, are not differentiated ; any or all 

 of the cells of the body may act as gametangia ; the sexual cells are 

 (except in the Conjugatse) free-swimming similar planogametes ; in the 

 Conjugatse the gametes are not set free into the water and they are not 

 ciliated. In the oogamous forms the sexual organs are antheridia and 

 oogonia ; they are more or less markedly differentiated. The sexual cells 

 are oospheres and spermatozoids. The oospheres are formed singly in the 

 oogonium. The spermatozoids are formed singly in the mother-cell ; they 

 resemble the zoospores of the respective plants, but are smaller and are 

 not green. 



In Ulothrix and (Edogonium, the contents of the spore, whether zygo- 

 spore 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 plant. In Coleochsete, the 

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

 several zoospores ; it is a small multicellular body, each cell of which 

 eventually sets free its contents as a single zoospore which, on coming to 

 rest, develops into a plant. 



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



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 and Zygnemeae ; 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 corresponding to itself ; hence the two 

 halves of a cell are of different ages. The only other mode of repro- 

 duction is by means of zygospores formed by the conjugation of two 

 individuals. On germination the contents of the zygospore divide into 

 two halves, each of which becomes an individual. 



Among the commoner forms of the Desmidiese are Closterium (Fig. 144), 

 Staurastrum, Euastf urn (Fig. 144 (7). They can best be found in pools of 

 peaty water 011 moors and bogs. 



Family 2. Zygnemece. These plants, consisting of long, delicate, un- 

 branched filaments composed of cylindrical cells, occur as floating green 

 masses in ponds and springs. Each cell contains a peripheral layer of 

 protoplasm in close contact with the cell-wall, enclosing a large central 



