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-bearing 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 individuals, 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 {Juglcms regia), and 

 has since been observed with the common Nut (Corylus 

 avellana). According to H. Miiller the individual 

 plants of a few hermaphrodite species differ in a like 

 manner; some being proterandrous and others pro- 

 terogynous-t On cultivated trees of the Walnut and 

 Mulberry, the male flowers have been observed to abort 

 on certain individuals,! 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 



Liguaggio Anemofilo.' Firenze, proterandrous and proterogynous 



1871. liermaphrodite individuals of the 



t Delpino, ' Ult. Oaservazioni same species, see H. Miiller, ' Die 



suUa Dioogamia,' part ii. fasc. ii. Befruchtung," &o., pp. 285, 339. 

 p. 337. Mr. Wetterhan and H. { ' Gardener's Chrou.' 1 847, pp. 



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



