THE LIVING ENVIRONMENT. 109 



gardener or forester can do with his ill-guided 

 footsteps, axe, spade, and knife can only be 

 appreciated by one who knows the habits of 

 plants. 



It is among the invertebrata, however, especially 

 insects and worms, that the most striking agents of 

 disease in plants are to be found, for, with the ex- 

 ception of certain rodents and we may logically 

 include also human invasions vertebrate animals 

 do not often appear in such numbers as to bring 

 about the epidemics and scourges only too com- 

 monly caused by insect pests. 



Insects injure plants in very various ways. 

 Some, such as locusts, simply devour all before 

 them ; others, e.g. caterpillars, destroy the leaves 

 and bring about all the phenomena of defoliation. 

 Others, again, eat the buds e.g. Grapholitha ; or 

 the roots e.g. wire-worms, and so maim the 

 plant that its foliage and assimilation suffer, or its 

 roots become too scanty to supply the transpira- 

 tion current. Many aphides, etc., p uncture the 

 leaves, suck out the sap, and pro duce deformations 

 and arr est ot leal-s urface, as well as act ual loss of 

 substance, and when numerous such insects induce 

 aTTthe evils of defoliation. Others, such as the 

 leaf-miners, tunnel into the leaves, with similar 

 results on a smaller scale. 



It must be remembered that a single complete 

 defoliation of a herbaceous annual, or even of a 

 tuberous plant like the potato, so incapacitates the 

 assimilatory machinery of the plant, that no stores 

 can be put aside for the seeds, tubers, etc., of 



