Significance of Sex and Nuclear Fusions. Harold Wager. J13 



organisation which takes place, in the absence of fertihsation, 

 at regular periods, and is sufficient for the continual develop- 

 ment of the organisms. 



These observations clearly indicate therefore that, although 

 nuclear fusion is necessary for the blending of hereditary cha- 

 racters, it is not essential for growth and development, since 

 the developmental stimulus under certain conditions can be 

 effected by other agencies. 



The observations which have recently been made on the bi- 

 nucleate cells in the vegetative tissues of the higher plants by 

 Prankerd (Ann. Bot. 1915), and Beer and Arber (Ann. Bot. 

 1915; Proc. R. Soc. 1919), have an important bearing on the 

 rejuvenation function of sexual and other nuclear fusions. It 

 appears that multinuclear cells are very widely distributed and 

 that they are characteristic of young tissues which are actively 

 carrying on the processes of life. Most frequently the cells are 

 binucleate, but three, four or even more may occur, and the 

 paired nuclei often become surrounded by a differentiated shell 

 of cytoplasm, "phragmosphere," which gradually expands until 

 it merges in the peripheral cytoplasm. Professor R. C. McLean 

 considers that there is also evidence of nuclear fusions taking 

 place in these cells. The multinucleate stage reaches its most 

 characteristic expression just previous to the maximum period 

 of growth, when metabolic activity is running high. There 

 appears to be in fact a definite cytoplasmic and nuclear re- 

 organisation in the cells of young tissues just at a time when 

 vigorous growth and development are taking place, and this 

 may "conceivably afford the organism a distinct advantage 

 in carrying out the chemical processes associated with growth, 

 and might tend to become perpetuated as a ^definite physio- 

 logical phase in the history of growing members." 



A normal sexual fusion includes at least two distinct phe- 

 nomena, (i) the blending of the parental characters derived 

 from two distinct lines of descent, and (2) rejuvenescence of 

 the reproductive cell by means of which it receives a new "stimu- 

 lus to growth and division. This exogamic binary sexual fusion 

 is found at the present day, so far as we know, in a few fungi 

 only, although formerly it may have been of more frequent 

 occurrence. In the majority of fungi in which binary sexual 

 fusion occurs {e.g. fusion of differentiated gametes) this fusion 

 is endogamous {e.g. the gametes are produced on the same in- 

 dividual). Here it is obvious that, since there can be no blending 

 of two lines of descent, the only purpose of this sexual fusion is 

 rejuvenescence. 



The production of distinct male and female organs on the bi- 

 sexual thalli of such forms as Cystopus and Peronospora indi- 



