vii Relations Between Plants and Animals 139 



are infested by thousands of these insects. Besides 

 sapping the strength of the plant, and choking the leaves 

 with honey-dew, the Aphides by their secretion make it 

 easier for injurious parasitic fungi to establish themselves 

 upon the leaves. On the other hand, they attract ants, 

 whose presence in moderate numbers at any rate may be 

 useful to the plants. 1 But as Herr Biisgen calculates that 

 in one case the quantity of carbo-hydrate material absorbed 

 by the Aphides from a plant was about one-sixth of that 

 required to furnish the whole foliage, we must agree with 

 him that this is too high a price to pay for problematical 

 benefits. 



The Ants and Aphides must serve as types of the 

 injurious insects, between which and plants there are 

 numerous interesting relations. We have to consider how 

 far various structures of plants, such as hairy stems, viscid 

 stems, pendent flowers, and the like, serve to save plants 

 from their enemies, as may be true in the case of unwel- 

 come ants ; we have also to notice what changes the 

 injurious insects, such as corn -insects, Phylloxera, Weevils, 

 etc., may effect on the plants which they infest ; and we 

 must also observe how the hostile insects, which affect 

 forest trees and vegetation generally, may occasion changes 

 which have far-reaching influences on the fauna, flora, 

 scenery, and even climate of a country-side. Readily 

 available information will be found in Miss E. A. Ormerod's 

 valuable work on Injurious Insects (second edition, Lond. 

 1891), while the more advanced student would do well to 



1 With all respect to this observer, one may still maintain 

 that during prolonged fine weather, especially in June, while leaf 

 tissues are fresh and young, and Aphides not yet abundant, such an 

 excess of assimilation sometimes takes place as to create an overflow 

 of nectar from leaves; the very ferns sometimes showing this with- 

 out Aphides upon them, or at any rate in adequate numbers. 



