6 DISEASES OF TREES 



plants all die a natural death, or whether they, at least in part, 

 succumb to external influences that is to say, are subject only 

 to accidental death. 



Experience teaches that, at any rate among the more highly 

 developed plants, each individual dies sooner or later, but that in 

 the case of perennial plants, particularly trees and shrubs, the 

 cause of death is always to be found in unfavourable external 

 influences. In the case of the more lowly organisms, which only 

 multiply by division and as yet exhibit no sexual reproduction, 

 one can scarcely speak of a natural death, because each part is 

 as old as the parent organism by the division, &c., of which it was 

 formed. Were a natural limit set to the life of a certain species 

 of plant which can only multiply by dividing, the result would 

 be that when this limit was reached every part, and therefore 

 also the offspring which had originated by division, would perish. 

 It is known, however, that this state of things does not exist. 

 In the case of those plants which are also reproduced by sexual 

 processes many different conditions are met with. In the case 

 of annual plants the vegetative part dies each year, and only the 

 embryos originating from the fertilized oospheres remain alive. 

 When, from these, plants capable of bearing seeds have deve- 

 loped, all that is preserved of them, in their turn, is the forma- 

 tive product arising from the sexual cells. Thus the vegetative 

 part of each plant dies owing to internal causes, though these, 

 in part, depend simply upon exhaustion consequent on the 

 formation of seed. We see then that natural death of the vege- 

 tative organs of the plant occurs from internal causes, whereas 

 the sexual cells only die if they have not been fertilized, or if, 

 owing to external causes, the product of fertilization has not 

 given rise to a new plant. Upon the unlimited duration of the 

 life of this part of the plant that is to say, of those sexual cells 

 which do not fall victims to accidental death depends indeed 

 continuity in the organic world, in other words, the development 

 and preservation of the vegetable and animal kingdoms. 



In the case of perennial plants it is only certain parts that 

 succumb to natural death each year. Amongst herbaceous 

 plants, for instance, it is the parts above ground which thus die 

 off: in the case of deciduous trees and shrubs, it is the outer 

 cortical tissues, the leaves, &c. 



