72 Practical Plant Biology. 



from the embryo, and fracture separates the plant from this 

 transitory organ of attachment. 



There are many points of interest raised in this sexual re- 

 production of Spirogyra. In the first place, what happens within 

 the mass formed by the two uniting cells ? Direct observation 

 gives us no clue. The mass is so dense and contracted that 

 the behaviour of the chloroplasts, much less that of the nuclei, 

 cannot be followed. Investigators, however, by using very refined 

 methods of killing conjugating cells at various stages of the 

 process and by using stains which colour differentially the 

 various parts of the cells have been able to give us an account 

 of these mysteries. The chloroplast of the cell which moves 

 across the conjugation-tube disappears apparently dissolved 

 or digested in the mass of protoplasm. The two nuclei come 

 into contact with one another and fuse into one. This single 

 nucleus divides immediately and the resulting two divide again. 

 Two of these suffer the fate of the chloroplast and the remaining 

 two fuse and produce a single nucleus for the zygospore. Thus 

 the zygospore finally contains the cytoplasm and the fused 

 fragments of the nuclei of the two cells and the chloroplast of 

 the cell which has been quiescent during the process. 



Evidently then the plant coming from the zygospore, or cell 

 formed by the sexual method of reproduction, carries material 

 derived from two individuals. This may affect the offspring in 

 two opposite ways. It has a two-fold possibility of inheriting 

 idiosyncrasies ; at the same time the idiosyncrasies of one parent 

 will tend to be nullified by those of the other. These differences 

 between sexual and asexual reproduction should always be borne 

 in mind. 



In the conjugation of Spirogyra two complete individuals 

 fuse the whole individual acting as a sexual cell and passing 

 complete into the zygospore, where its activity re-awakens in 

 the next generation. The process is apparently repeated in- 

 definitely so that, apart from accidents, we may regard the proto- 

 plasm of Spirogyra or Chlamydomonas (which in this respect is 

 similar) as immortal. It simply undergoes subdivision and 

 fusion. 



These two organisms further resemble each other 'in the fact 

 that the two cells which fuse in sexual reproduction are, ex- 

 ternally at least, similar to one another. It has been pointed 

 out that in the process of conjugation in Spirogyra one of the 

 fusing cells gametes exhibits more motion than the other and 

 is consequently often spoken of as the sperm or male cell, the 



