CHAP. X. MEANS OF CROSS-FERTILISATION. 391 



flowers of Lamium album which a humble-bee visited 

 had been already exhausted of their nectar. In order 

 that distinct plants should be intercrossed, it is of course 

 indispensable that two or more individuals should grow 

 uear one another ; and this is generally the case. Thus 

 A. de Candolle remarks that in ascending a mountain 

 the individuals of the same species do not commonly 

 disappear near its upper limit quite gradually, but 

 rather abruptly. This fact can hardly be explained 

 by the nature of the conditions, as these graduate away 

 in an insensible manner, and it probably depends in 

 large part on vigorous seedlings being produced only 

 as high up the mountain as many individuals can 

 subsist together. 



With respect to dioecious plants, distinct individuals 

 must always fertilise each other. With monoecious 

 plants, as pollen has to be carried from flower to flower, 

 there will always be a good chance of its being carried 

 from plant to plant. Delpino has also observed * the 

 curious fact that certain individuals of the monoecious 

 walnut (Juglans regia) are proterandrous, and others 

 proterogynous, and these will reciprocally fertilise each 

 other. So it is with the common nut (Corylus avellana),] 

 and, what is more surprising, with some few her- 

 maphrodite plants, as observed by H. Muller.t These 

 latter plants cannot fail to act on each other like 

 dimorphic or trimorphic heterostyled species, in which 

 the union of two individuals is necessary for full 

 and normal fertility. With ordinary hermaphrodite 

 species, the expansion of only a few flowers at the same 

 time is one of the simplest means for favouring the 

 intercrossing of distinct individuals; but this would 



* 'Ult. Osservazioni,' &c., part J 'Die Befntchtung ' &c. pt\ 

 li. fuse. ii. p. 337. 285, 339. 



t ' Nature,' 1875, p. 26. 



