96 



OUTLINES OF EVOLUTIONARY BIOLOGY 



nucleus (nu.\ is suspended by slender radiating protoplasmic 

 threads (sir.). So far, both in structure and arrangement of its 

 component cells, the plant closely resembles a single hair of 

 Tradescantia (compare Fig. 25). It differs, however, in the 

 presence in each cell of one or more 

 conspicuous chloroplastids or chroma- 

 tophores (cr.), coloured bright green 

 by chlorophyll and wound spirally 

 round and round inside the cell-wall, 

 like pieces of ribbon. It is from 

 these characteristic structures that 

 the name Spirogyra is derived. 



The filaments increase in length by 

 transverse fission of the component 

 cells. Every cell, however, or to speak 

 more accurately its protoplasmic con- 

 tents, must also be looked upon as 

 a potential gamete. Conjugation in 

 some species takes place between the 

 cells of two filaments which are lying 

 side by side, parallel with one another 

 (Figs. 43, 44). In others it takes place 

 between adjacent cells of the same 

 filament, but we may confine our 

 attention to the former case. The 

 first indication of the process is seen 

 in the formation of a small, hollow r pro- 

 tuberance on the wall of one of a pair 

 of cells which happen to be more or 

 F IG . 43. _ Conjugation in less opposite to each other (Fig. 44, a.). 

 Spirogyra, showing in one This is shortly followed by the forma- 

 Filanient solitary Cells ,. , . ., , , 



(s.c.) which have failed tion of a similar protuberance on 

 to mate, and in the other the wall of the other cell (&.). The 

 Cftf. &&UL) 88 ' two protuberances meet (*.) and fuse 

 together, and the cell-walls at the 



point of union are dissolved away (d.). Thus a hollow canal is 

 formed placing the cavities of the two conjugating cells in free 

 communication with one another. In the meantime changes 

 are going on in the protoplasmic contents of the conjugating 

 cells, essentially similar in the two members of each pair but 

 with one cell still taking the lead and the other lagging somewhat 



