10 INTEODUCTION. 



of the other sex, the difference between the two kinds 

 is usually not great. When the difference is great, 

 as we see in catkin-hearing plants, this depends 

 largely on many of the species in this, as well as in 

 the next or dioecious class, being fertilised by the 

 aid of the wind;* for the male flowers have in this 

 case to produce a surprising amount of incoherent 

 pollen. Some few monoecious plants consist of two 

 bodies of indi-viduals, with their flowers differing in 

 function, though not in structure ; for certain indivi- 

 duals mature their pollen before the female flowers on 

 the same plant are ready for fertilisation, and are called 

 proterandrous ; whilst conversely other individuals, 

 called proterogynous, have their stigmas mature before 

 their pollen is ready. The purpose of this curious func- 

 tional difference obviously is to favour the cross-fertili- 

 sation of distinct plants. A case of this kind was first 

 observed by Delpino in the Walnut {Juglans regfia), and 

 has since been observed with the common Nut {Gorylus 

 avellana). I may add that according to H. Miiller the 

 individuals of some few hermaphrodite plants differ in 

 a like manner ; some being proterandrous and others 

 proterogynous.t On cultivated trees of the Walnut and 

 Mulberry, the male flowers have been observed to abort 

 on certain individuals J, which have thus been converted 

 into females ; but whether there are any species in a 

 state of nature which co-exist as monoecious and female 

 individuals, I do not know. 



The third Class consists of dioecious species, and the 



* Delpino, ' Studi sopra uno xi. p. 507, and 1875, p. 26.* On 



Lignaggio Anemofilo.' Pirenzo, proterandrous and proterogynona 



1871. hermaphrodite individuals of tho 



t Delpino, ' tJlt. Osservazioni same speeies, see H. Miiller, ' Die 



Bulla Dicogamia,' part u. fasc. ii. Befruohtung,' &o., pp. 285, 339. 



p. .337. Mr. Wetterhan and H. J 'Gardener's Cbron.' 1847, pp. 



Miiller on Corylus, ' Nature,' vol. 541, 558. 



