INTRODUCTION 7 



The plant-individual proper, however, only dies in consequence 

 of unfavourable external influences. As a matter of fact, every 

 tree is rejuvenated each year by the cambium forming new 

 tissues at its periphery, and by new shoots and buds. It is a 

 matter of experience that the duration of the life of all trees is 

 limited, but it is not proved whether this is to be ascribed to in- 

 ternal causes, or is the result of the innumerable influences which 

 act more or less prejudicially on the plant from without. The re- 

 duction and final cessation of the growth in height of a tree, after 

 attaining a certain maximum, must be ascribed to interference 

 with the factors of nutrition,. and, in all probability, especially 

 to the fact that the forces which conduct the water and nutritive 

 materials to the highest bud of the tree are limited in their 

 action, and that sooner or later, depending on the specific and 

 individual nature of the plant, these no longer suffice to provide 

 for the continuance of growth in height. If we cut a slip from 

 an old tree, it will pursue the same cycle of development as the 

 parent tree, thereby proving that by vegetative multiplication 

 the life of a plant may be indefinitely prolonged. Hitherto no 

 phenomenon has been discovered from which one may conclude 

 with certainty that internal natural causes of death are peculiar 

 to all, or even to any perennial plants. In this connection the 

 question is at once suggested whether " the feebleness of old 

 age " is a factor which must be regarded at all in considering the 

 diseases of plants. In discussing how diseases arise we shall 

 show that old age, quite as well as youth, may predispose a 

 plant to some disease or other. In itself, however, the feebleness 

 of old age is not a natural condition attributable to internal 

 causes, but is a state induced by external influences. The older 

 a tree is, so much the more numerous are the dangers through 

 which it has had to pass, and so much the greater is the 

 number of its injuries and wounds through which parasites 

 and saprophytes can find an entrance into its interior. Again, 

 the older a tree is, the narrower are its annual rings, and 

 with so much the more difficulty and tardiness does it suc- 

 ceed in occluding a wound. Finally, the older a tree is, 

 the more sluggish are its nutritive processes, because, on the 

 one hand, the soil in which the roots are fixed has become 

 denser, thereby impeding the entrance of air, and, on the other 



