374 



BOTANY OF THE LIVING PLANT 



protoplasms contract : one of them contracts earlier than the other ; 

 it then passes bodily through the now open tube, and its cytoplasm 

 coalesces with that of the other cell, while its chloroplast becomes dis- 

 organised (Fig. 277). The nuclei remain for a time distinct. The zygote 

 changes to a reddish colour, fats are stored in it, and a thickened wall 

 is formed. Freed from the parent filaments it remains dormant. 

 On germinating the outer wall ruptures, while the inner covers the 

 enlarging protoplast. The nuclei fuse, and later undergo tetrad- 

 division, with reduction. But only one of the four survives as the 

 nucleus of the cell from which, by division, the new haploid filament 

 arises. 



In Mesocarpus the conjugation is similar, except that the zygote 

 is formed actually in the tube connecting the conjugating cells. 



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

 zygote on germination has been followed (Fig. 278), and there also, after 



1. 



2. 



4- 5- 6. 



Fig. 278. 



Syngamy and germination of a Desmid, Closterium, after Klebahn. 1, zygote before 

 nuclear fusion ; 2, first nuclear division . 3, binuclear stage ; 4, second nuclear division ; 

 5, two cells, each with a small and a large nucleus ; 6, formation of two new Closterium 

 cells each with one large nucleus ; the other is disorganised. (From Oltmanns.) 



delayed fusion, the fusion-nucleus divides first 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 Conjugales 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 class of unicellular Algae with very 

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



