274 The Living Plant 



individuals being preserved for breeding in each generation. 

 And perhaps it will yet be found that plants can be rendered 

 immune by acclimatization, so to speak, to their diseases, as 

 animals can be to theirs. But of these matters we know little 

 as yet. 



We come finally to the last of the environmental factors hostile 

 to plants, and that is the depredations of animals. These, in- 

 deed, cannot be otherwise than constant and great, since in the 

 long run every jot of the food that is eaten by animals has to be 

 ravaged from plants. The general defense of plants, however, is 

 passive and indirect, consisting chiefly in a reliance upon their 

 own superabundant powers of growth, regeneration, and repro- 

 duction, in which features they surpass animals many fold. So 

 great are these powers, indeed, that plants are enabled to produce 

 organic material in vast excess of their own needs, upon which 

 fact depends the very possibility of the existence of animal life. 

 And it may be true that the use of this surplus by animals is not 

 only of no damage to plants, but may even be useful in removing 

 superfluous material and making room for a more active evolu- 

 tion of that which remains. In any case it is a generous payment 

 for the various services which animals render to plants. 



Although plants thus appear to possess few adaptations against 

 animal attacks, especially in their vegetative parts, there appear 

 to be notable exceptions. Thus, a good many herbs develop 

 various substances in their stems or their leaves, bitter oils, 

 turpentiny resins, acrid glucosides, astringent tannins, alkaloids, 

 or even needle-pointed irritating crystals, which render those 

 plants distasteful to herbivorous animals, including all kinds 

 from the greatest of beasts down to slugs and innumerable in- 

 sects. Everybody has noticed the clumps of such plants left 

 standing quite isolated in pastures by cattle which browse the 

 grass well-nigh to the roots all around them. In these cases, 

 however, the protection is perhaps incidental rather than adapt a- 

 tional, and may be defensive against Fungi rather than animals; 



