GROUP I. THALLOPHYTA : ALG.^. 247 



Vegetative multiplication by division occurs in some of the lower 

 forms (e.g. Protococcoidese) of this sub-class. Reproduction by 

 zoospores is general (absent in Pleurococcacese, Conjugate, most 

 Volvocoidese, Characeae) ; they are formed, not in specialised re- 

 productive 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 piano- 

 gametes or aplanogametes (Conjugatse), 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 

 Characese and that of some species of (Edogonium), 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 

 Sphaeroplea ; the unicellular (as also the coenocytic) antheridium 

 gives rise to numerous spermatozoids, except in Coleochsete and in 

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

 cellular antheridium of the Characese, numerous spermatozoids 

 are developed singly in distinct mother-cells. 



There is considerable polymorphism in many members of this 

 sub-class, so that various forms which were considered to be inde- 

 pendent members of the simpler families are now known to be 

 merely phases in the life-history of more complex forms ; for in- 

 stance, various unicellular forms, such as Protococcus, Palmella, 

 Grlceocystis, etc., formerly classed among the Protococcacese, are 

 now known to be stages in the life-history of other Protococcoidese, 

 Confervoidese, Siphonoidese, 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 then isogamous with 

 conjugation of planogametes. 



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

 multicellular not filamentous ; not attached, motile by means of cilia ; 

 the body is a thallus, with limited growth ; reproduction, vegetative 

 by division, asexual rarely by zoospores, sexual, either isogamous 

 with conjugation of planogametes, or oogamous. 



