GREEN ALGAE 401 



Conjugation in the Desmids is essentially similar. The behaviour of the 

 zygote on germination has been followed (Fig. 3^8), and there also, after 

 delayed fusion, the nucleus divides into two and then into four ; but two are 

 atrophied, while the others remain as the nuclei of the two new cells formed 

 on germination. Thus in the Conjugatae there is a tetrad-division, and a 

 presumable reduction which follows on conjugation, just as tetrad-division 

 follows on sexual fusion elsewhere. 



It is different in the Diatoms, a distinct family of unicellular Algae with very 

 many forms, marine and fresh-water, great numbers of which are found in the 

 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 twq 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, first 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 Conjugatae it follows. It 

 may then be concluded that the vegetative period of the Conjugatae is 

 haploid , while that of certain Diatoms is diploid ; as it is also in Fucus. Such 

 facts have their importance in questions to be discussed in Chapter XXXII. 



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 such as the 

 Flagellates : 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 distinction of male and female gametes 

 correspond in the Volvocales, Ulothricales, and Siphonales, though 

 these series are sharply distinct in vegetative structure. They 

 also resemble the progression already traced in the still more distinct 

 Brown Algae (p. 385). 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, (Edogonium, Vaucheria, and Fucus 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. 



B.B. 2C 



