THE GENUS ROSA 265 



with its relatives; it may, therefore, cross freely with them. 

 Nevertlieless, one feature shuts it off from them. Its 

 curiously lower habit of growth renders it an unfit subject for 

 the hedges and thickets in which its nearest relatives revel ; it 

 delights in forming a low scrub of compact little bushes in 

 deserted localities. Pollination, then, if the microgene is at 

 all prone to allogamy, will most likely take place through the 

 agency of low-flying insects which, only in rare instances, will 

 have visited other EticanincE, Afzeliance. and the like ; there- 

 fore contamination, although not prevented, is at a minimum. 



Rosa cinnamohiea and R. rugosa need not detain us long. 

 Both are forms isolated geographically and otherwise, 

 R. ciiinamomea being Arctic and Alpine and possessing no 

 closely allied relatives, and R. rugosa being a native of Japan. 

 Seclusion even more powerful than with R. pimpinellifolia is 

 at work to prevent their crossing with other forms, and thus 

 their condition harmonises with that of that species. 



If our explanation of the superior genetic purity of these 

 forms, as deduced from the state of their pollen, is warranted, 

 a necessary corollary immediately arises. Monotypical 

 genera and species completely secluded by any circumstance 

 whatever from outside influence should invariably produce 

 perfectly formed pollen grains. To test this I have prepared 

 and examined pollen of wild Narcissus pseudo-narcissus (from 

 an isolated colony growing on the heather in Earndale, North 

 Yorks, far away from any cultivated form), Pyrus Aucuparia 

 (lilcewise from the moors) Primula farinosa, Trientalis europaea, 

 Ulex europaeus and Paruassia palustris, all far removed from 

 the disturbing influences of allied species. In every case the 

 microspores were absolutely perfect. 



The circumstances attending Pyrus Aucuparia are particu- 

 larly interesting. In Europe it is an isolated species, but in 

 Canada it has several related representatives like Pyrus 

 americana and P. sambucifolia, and in these the pollen is far 

 from normal. 



