264 DR. J. W. HF.SI.OP HARRISON ON 



and its habits, to extract, if possible, any clue to their com- 

 parative immunity. Amongst them four sections are repre- 

 sented, 7?. pimpinellifolia belonging to the Pimpifiellifolice, 

 R. mollis and R. omissa to the Villosce, R. senticosa to the 

 EucaniiKz^ and R. rugosa and R. ciiinamomea to the Cinna- 

 momece. 



Rosa pimpinellifolia is the sole representative of its group 

 within our islands ; whatever variability it displays is slight, 

 and no forms attaining the rank of a microgene exist. Again, 

 its flowering period is manifestly earlier than its congeners' ; 

 furthermore, it prefers stations abhorred by its relatives, wind- 

 swept moors, mountain scars, coast ravines, cliffs and sandhills 

 being its chosen habitats. Hence, geographically, and in 

 habit, R. pimpinellifolia is powerfully secluded from extra- 

 specific influences, so that almost inevitably its purity is 

 ensured to a very considerable extent. However, its seclusion 

 is not perfect ; in many northern localities it encounters 

 R. mollis and R. omissa^ not only geographically, but also in 

 season ; more rarely it meets with the Eucanince and 

 AfzeliancE. Hybridity with all of these is not excluded, but so 

 remote are its affinities from them that almost uniformly its 

 hybrids are patent and non-fertile ; they cannot perpetuate 

 their kind, and do not form a significant factor in our rose 

 flora. Their rarity is even more pronounced in Central 

 Europe where R. pimpinellifolia rarely collides with possible 

 partners. 



R. mollis and R. omissa are similarly isolated. Northern 

 in range, and early in flower, their purity is, in general, main- 

 tained but is far from being absolute. They can intercross to some 

 extent, but only slightly, with the Encaiiince and other groups, 

 so that the Villosce, in the main, produce perfect pollen. Still, 

 forms like R. caenilea, undeniably a member of the VilloscB, 

 do crop up and hint by outward characters, as well as in their 

 pollen, at their development from previous crossing. 



Rosa sen/iiosa, however, proves more difficult of solution. A 

 t\pical Eucanine, it grows in the same districts and flowers 



