118 



THE AMERICAN NATURALIST 



[Vol. LIII 



fertile-cell form, with porticalis spore form; ahc, typical 

 porticalis; aBc, porticalis dimensions and spore form, 

 with varians fertile-cell form; ahC, porticalis dimensions 

 and fertile-cell form, with varians spore form; aBC, por- 

 ticalis dimensions, with varians fertile-cell and spore 

 form. 



The female filaments derived from these several zygo- 

 spores should show these eight possible combinations 

 when they fruit. These forms have all been found in the 

 two series of collections in each of two successive years, 

 and there seems to be no question but what they are the 

 products of hybridization and segregation. Since the in- 

 heritance is entirely matriclinal, segregation occurs in 

 the first generation. 



As to the relative numbers of the several types it is 

 impossible to count filaments in a collection. Theoreti- 

 cally they should be present in about equal numbers if all 

 the filaments are of hybrid origin. In all the collections, 

 however, there were filaments conjugating in the usual 

 way, so that any attempt at counting filaments would be 

 useless even though it were practically possible. 



Summary 



1. Hybridization between Spirogyra communis and 8. 

 varians and between S. varians and S. porticalis have 

 been observed. The forms probably derived from these 

 crosses have also been found. 



2. Hybrids are exceedingly rare among species of 

 Spirogyra. 



3. They have been observed only in a few species. 



4. Hybrid zygospores may be formed between species 

 even though they have very different vegetative and spore 

 characters. 



5. The nuclei derived from the two gametes do not fuse 

 until after tlie zygospoiv wall matures. The form of the 

 zvu-()>iHMv .h.t.Tniincd ..ntiiviv I.v the tVitiah^ oaniclc. 



a The fn>ioii mxr\vn< of a hybrid zyg(..,H.,v (•(uitains 

 factors for all the various characters of both species. 



