CHAP. X. MEANS OF FERTILISATION. 357 



of seeds. These lists have been compiled from the 

 several previous tables, with some additional oases from 

 my own observations and those of others. The species 

 are arranged nearly in the order followed by Lindley 

 in his 'Vegetable Kingdom.' The reader should 

 observe that the sterility or fertility of the plants in 

 these two lists depends on two wholly distinct causes ; 

 namely, the absence or presence of the proper means 

 by which pollen is applied to the stigma, and its less 

 or greater efficiency when thus applied. As it is 

 obvious that with plants in which the sexes are separate, 

 pollen must be carried by some means from flower to 

 flower, such species are excluded from the lists ; as 

 are likewise heterostyled plants, in which the same 

 necessity occurs to a limited extent. Experience has 

 proved to me that, independently of the exclusion of 

 insects, the seed-bearing power of a plant is not 

 lessened by covering it while in flower under a thin 

 net supported on a frame; and this might indeed 

 have been inferred from the consideration of the two 

 following lists, as they include a considerable number 

 of species belonging to the same genera, some of which 

 are quite sterile and others quite fertile when protected 

 by a net from the access of insects. 



List of Plants which, when Insects are excluded, are either 

 quite sterile, or produce, as far as I could judge, less 

 than half the number of Seeds produced by unpro- 

 tected Plants. 



Passiflora alata, racemosa, cosrulea, edulis, laurifolia, and some 

 individuals of P. quadrangularis (Passifloracese), are quite 

 sterile under these conditions: see 'Variation of Animals 

 and Plants under Domestication,' chap. xvii. 2nd edit. vol. ii. 

 p. 118. 



Viola canina (Violacese). Perfect flowers quite sterile unleai 

 fertilised by bees, or artificially fertilised. 



