GREEN ALGAE 375 



floating " Plankton." Their chloroplasts are brown, and the uninucleate 

 protoplast is enclosed between two silicified shells with delicate sculpturing, 

 which fit over one another like the two parts of a pill-box. Vegetative 

 division results in regular decrease in size of the cells, till a limit is reached, 

 from which recovery is usually by conjugation, resulting in Auxospores. The 

 nuclei of the conjugating cells of certain types, such as Rhopalodia, hrsc divide 

 into four : each cell then divides into two gametes with two nuclei in each 

 of them, one large and one small : the gametes then fuse in pairs, the larger 

 nuclei also fusing, while the smaller disintegrate. Here also there is a tetrad- 

 division, but it precedes conjugation, while in the Conjugales it follows. It 

 may then be concluded that the vegetative phase of the Conjugales is haploid, 

 while that of certain Diatoms is diploid. These facts have their importance 

 in questions to be discussed in Chapter XXXIII. 



Examples such as these from the Green Algae show how diverse 

 those plants are in structure and in propagative method. It may 

 be held that they sprang from some common source, including motile 

 and non-motile forms, such as the Protophyta : and this would seem 

 probable from their characters, both vegetative and propagative. 

 But the differences which they show suggest a plurality of lines of 

 parallel development. More especially does this emerge from their 

 comparison in respect of sexual differentiation. The steps of distinc- 

 tion of male and female gametes correspond in several large Orders, 

 e.g. Volvocales and Ulotrichales, though these series are sharply 

 distinct in vegetative structure. The only possible conclusion from 

 such facts is that the distinction of the sexes has been achieved not 

 only once, but in a number of distinct evolutionary series. There has, 

 in fact, been parallel development, or as it is styled, homoplasy. The 

 spermatozoids and ova of Volvox, Oedogonium, and Vaucheria are not 

 then to be held as homogenous, that is, produced from a common 

 ancestry that bore spermatozoids and eggs ; but homoplastic, that is, 

 each has arrived as a result of independent sexual evolution from 

 some ancestry which had not male spermatozoids or female eggs, but 

 undifferentiated gametes as the propagative organs. 



While a majority of Green Algae are typically haploid organisms, 

 and undergo reduction at the first nuclear division in the zygote, 

 there are others, e.g. among the Siphonales, which are typically 

 diploid organisms. Thus the Green Algae give some indication of the 

 very considerable diversity of life-cycle which is characteristic of the 

 Algae as a whole. This aspect is discussed in greater detail at the end 

 of Chapter XXIII. 



