26o DR. J. \V. HESLOP HARRISON ON 



be set in motion were those attacking the matter from a 

 cytological standpoint followed by an examination of the 

 pollen conditions in the genus. Only the latter research will 

 be discussed here in view of the enormous amount of material 

 which has accumulated as a result of the earlier enquiry. 



Very early indeed in the study of plant hybrids was their 

 sterility observed, and determined in general to depend on 

 their badly developed pollen. With the recognition of this 

 fact, naturally enough, imperfect pollen was soon regarded as 

 an adequate test of hybridity, as Dutrochet insisted in 1832. 

 iVIoreover, Gaertner elaborated the point a little later, and 

 stated it in terms differing but slightly from those current to- 

 day. To test the reasonableness of these opinions is far from 

 difficult. If one takes pollen from known natural hybrids 

 like those between Liiiaria repeiis and Z. vuIc^aris{=zL. sepii/in), 

 between Rt/bus idaeus and R. caeshis, between Rosa viollis and 

 R. pimpinellifolia, between- Primnla officinalis and P. acaitlis, 

 and examines it microscopically, a huge proportion, if not the 

 whole of the grains, is found to be malformed or shrivelled, 

 and thus shows a conspicuous lack of cytoplasmic contents. 

 Not five minutes ago I plucked a wild hybrid between the two 

 Primulas and was astounded to find that quite 90 per cent, of 

 its pollen was in this state -an extraordinary fact when one 

 realises the usual conception of the relationship between the 

 parents and one's easy assumption that, if any hybrid is com- 

 pletely fertile, this one is. 



With these facts as a basis the contents of the pollen sacs 

 of certain rose microgenes were critically examined, when 

 they, too, showed in most cases the same type of pollen 

 abortion, although the proportion of sound grains varied with 

 the form. One of the worst offenders in this respect appears 

 io he Rosa mollis \s.r. caernlea (text fig. 2, c) ; a little better 

 behaviour is displayed by Rosa flexihilis (text fig. 2, h) and so 

 on until we reach Rosa rugosa (text fig. 2, a) when practically 

 every grain is perfect. 



