GROUP I. THALLOPHYTA: ALGJ: : CHLOROPHYCE^:. 235 



specialised reproductive organs, but in any cell or part of the body. 

 A sexual process has been observed in members of every division 

 of this sub-class : it is either isogamous, consisting in the fusion of 

 planogametes or aplanogametes (Conjugates), with the formation 

 of a zygospore ; or oogamous, consisting in the fertilisation of an 

 oosphere, which is in no case extruded from the female organ, by 

 a spermatozoid, with the formation of an oospore. The sexual 

 organs are either gametangia, or antheridia and oogonia ; they are 

 unicellular in all the cellular forms (except the antheridium of 

 Characea3 and that of some species of CEdogonium), and present 

 various degrees of specialisation. A gametangium gives rise to 

 many planogametes, but to not more than one aplanogamete ; the 

 oogonium produces but a single oosphere, except in the coenocytic 

 Sphaaroplea; the unicellular (as also the coenocytic) antheridium 

 gives rise to numerous spermatozoids, except in Coleochrete and in 

 some species of CEdogonium where it forms only one; in the multi- 

 cellular antheridium of the Characese, numerous spermatozoids 

 are developed singly in distinct mother-cells (see p. 115). 



In those Chlorophyceae in which both sexual and asexual spore- 

 formation takes place, the life-history generally presents an alter- 

 nation of generations.. There is, in addition to this, considerable 

 polymorphism in many members of the sub-class, so that various 

 forms which were considered to be independent members of the 

 simpler families are now known to be merely phases in the life- 

 history of more complex forms ; for instance, various unicellular 

 forms, such as Protococcus, Palmella, Gloeocystis, etc., formerly 

 classed among the Protococcaceaa, are now known to be stages in 

 the life-history of other Protococcoidea3, Confervoideae, Siph- 

 onoideee, etc. 



The Chlorophyceae may be classified as follows : 



Series I. Protococcoidese : plants unicellular, isolated or held together by 

 mucilaginous cell-walls into colonies ; non-motile; the body is a thallus, 

 and has no apical growth ; reproduction, vegetative by division, asexual by 

 zoospores, rarely sexual and isogamous with conjugation of planogametes. 



Series II. Volvocoidese : plants unicellular or multicellular, and when multi- 

 cellular not filamentous ; not attached, motile by means of cilia ; the body 

 is a thallus, with limited growth ; reproduction, vegetative by division, 

 asexual by zoospores, no zoogonidia ; sexual, either isogamous with con- 

 jugation of planogametes, or oogamous. 



Series III. Siphonoidece : plants cttiiocytic, unseptate or incompletely septate ; 

 noo -motile ; the body may be a thallus or may be differentiated into stem, 

 leaf, and root ; with or without apical growth ; reproduction, asexual by 



