CHAPLER XX. 

 HAPLOID ORGANISMS AND MUTATION. 



For the investigation of the existence or non-exis- 

 tence of mutation, no group of plants offers better op- 

 portunities than mosses. 



Bacteria are all too uncertain for such investiga- 

 tions on account of the possibility of contamination, 

 especially if great care is not taken to start with abso- 

 lute certaintly from a single cell. The delution-method, 

 so frequently ressorted to, never gives absolute cer- 

 tainty in this respect. 



Organisms with multinucleate cells are never safe 

 objects because Burgeff has shown, in his most interes- 

 ting investigations on the results of crossing different 

 t37pes of Phycomyces that such cells may be hetero- 

 caryotic e. g. may contain nuclei of different constitu- 

 tions, so that with such polyenergid organisms one can 

 at best, obtain the same relative certainty for their 

 specific purity, as is obtainable in the case of diploid 

 organisms, but no greater certainty. 



But mosses offer very much better opportunities for 

 the final settlement of the vexed mutation-question. 



As each mossplant arises from a single gamete, it can 

 notbeaninter-gametic hybrid as diploid organisms so 

 frequently are. Leaving for the present the question 

 of the possibility of intragametic hybrids aside, we 



