10 INTRODUCTION. 



of the other sex, the difference between the two kinda 

 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 tho 

 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 (Juglans regia), and 

 has since been observed with the common Nut (Corylus 

 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.f 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 



Lignaggio Anemotilo.' Firenze, proterandrous and proterogyuous 



1871. hermaphrodite individuals of tho 



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



sulla Dicogamia,' part ii. fasc. ii. Befruolitung," &c., pp. 285, 339. 



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



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



