392 MEANS OF CROSS-FERTILISATION. CHAP. X. 



render the plants less conspicuous to insects, unless 

 the flowers were of large size, as in the case of several 

 bulbous plants. Kerner thinks * that it is for this 

 object that the Australian Villarsia parnassifolia pro- 

 duces daily only a single flower. Mr. Cheeseman also 

 remarks,! that as certain Orchids in New Zealand 

 which require insect-aid for their fertilisation bear only 

 a single flower, distinct plants cannot fail to intercross. 

 So it is with the American species of Drosera,t and, as 

 I hear from Professor Caspary, with water-lilies. 



Dichogamy, which prevails so extensively throughout 

 the vegetable kingdom, much increases the chance of 

 distinct individuals intercrossing. With proterandrous 

 species, which are far more common than pro- 

 terogynous, the young flowers are exclusively male 

 in function, and the older ones exclusively female; 

 and as bees habitually alight low down on the spikes 

 of flowers in order to crawl upwards, they get dusted 

 with pollen from the upper flowers, which they carry 

 to the stigmas of the lower and older flowers on 

 the next spike which they visit. The degree to which 

 distinct plants will thus be intercrossed depends on 

 the number of spikes in full flower at the same time 

 on the same plant. With proterogynous flowers and 

 with depending racemes, the manner in which insects 

 visit the flowers ought to be reversed in order that- 

 distinct plants should be intercrossed. But this whole 

 subject requires further investigation, as the great 

 importance of crosses between distinct individuals, 

 instead of merely between distinct flowers, has hitherto 

 been hardly recognised. 



* ' Die Schutzmittel,' &c. p. 23. work, in ' American Journal of 



t ' Transact. New Zealand In- Science,' vol. xiii., Feb. 1877, p. 



ptitute,' vol. v. 1873, p. 356. 135. 

 J Asa Gray, in a review of this 



