876 MEANS OF CtlOSS-FERTILISATION. Chap. X. 



habitually visit the flowers ; this has been well shown 

 by H. Muller by his comparison of lowland species 

 which are chiefly visited by bees, with alpine species 

 belonging to the same genera which are visited by 

 butterflies.* Flowers may also be adapted to certain 

 kinds of insects, by secreting nectar particularly attrac- 

 tive to them, and unattractive to other kinds ; of which 

 fact Epipadis latifolia offers the most striking instance 

 known to me, as it is visited exclusively by wasps. 

 Structures also exist, such as the hairs within the 

 corolla of the foxglove (Digitalis), which apparently 

 serve to exclude insects that are not well fitted to bring 

 pollen from one flower to another, f I need say nothing 

 here of the endless contrivances, such as the viscid 

 glands attached to the pollen-masses of the Orchideae 

 and Asclepiadas, or the viscid or roughened state of 

 the pollen-grains of many plants, or the irritability of 

 their stamens which move when touched by insects, &c, 

 as all these contrivances evidently favour or ensure 

 cross-fertilisation. 



All ordinary flowers are so far open that insects can 

 force an entrance into them, notwithstanding that 

 some, like the Snapdragon (Antirrhinum), various 

 Papilionaceous and Fumariaceous flowers, are in 

 appearance closed. It cannot be maintained that their 

 openness is necessary for fertility, as cleistogamic flowers 

 which are permanently closed yield a full complement 

 of seeds. Pollen contains much nitrogen and phos- 



* 'Nature,' 1874, p. 110; 1875, &c. protect the flowers from the 



p. 190 ; 1876, pp. 210, 289. access of crawling or wingless 



t Belt, 'The Naturalist in Ni- insects, which would steal the 



caragua,' 1874, p. 132. Kerner nectar, and yet, as they do not 



has shown in his admirable essay, commonly carry pollen from one 



' Die Schutzmittel der Bliithen plant to another, but only from 



gegen unberufene G'aste, 1826,' flower to flower on the same 



that many structures hairs, viscid plant, would confer do benefit to 



glauds, the position of the partsv the species. 



