138 Chapters in Modern Botany CHAP. 



sciences there branch off innumerable pleasant byways, 

 each leading into a tiny world of its own, a minor infinity ; 

 at first sight no doubt a hazy labyrinth, yet on deepening 

 study an ordered microcosm of evolutionary law. 



Plants and Aphides. From ants we naturally pass to 

 the plant-lice or Aphides, for these little insects which form 

 sweet juice receive much attention from the ants, who 

 sometimes use them as cows. 1 Many of them, such as 

 those which infest roses, fruit-trees, and hops, are exceed- 

 ingly injurious to the plants, for they suck the sap, choke 

 the pores of the leaves with their honey-dew, and do other 

 damage. The honey-dew, of which the ants are so fond, 

 has been for long the subject of much discussion. Pliny, 

 and many later naturalists who should have known better, 

 said that it fell from heaven ; many have described it as 

 an exudation from plants, while other perhaps most 

 naturalists speak of two kinds an animal honey -dew 

 formed by the Aphides, and a vegetable honey-dew exuded 

 in some way or other from the plants. 



Biisgen's recent observations 2 lead him to affirm con- 

 clusively that all honey-dew, excepting sugary exudations 

 caused by parasitic fungi, is an excretion of the Aphides. 

 The only flow of sap from the cells of the plants is into 

 the mouths of the insects ; the sweet juices are slightly 

 changed in the food-canal, but the quantity of glucose in 

 this is out of proportion to the animal's wants, and hence 

 what is unused as food passes out. As a single aphis may 

 form as many as forty-eight drops of honey-dew in twenty- 

 four hours, it is not surprising that a very rain of nectar 

 should sometimes fall from trees (especially limes) which 



1 See conveniently J. Arthur Thomson, The Study of Animal 

 Life. Lond. 1892. 



2 " Das Honig-Tau," Jenaiscke Zeitschrift fur Naturwissenschaft, 

 vol. xxv. (1891), pp. 339-428. 2 Pis. 



