INTRODUCTION 11 



substance due to the exudation of turpentine. It is not until 

 after the development of duramen, which no longer conducts 

 water, that a predisposition for wood-diseases sets in, because 

 now, when a branch is broken off, the inner wood no longer 

 protects itself against attack by pouring out turpentine, for it 

 is only in the watery alburnum that turpentine and resin are 

 forcibly pressed out of the resin-canals. In the case of trees 

 advancing age is also, as a rule, accompanied by a diminution 

 in the breadth of the successive annual rings, and the result 

 of this is that wounds are not so quickly occluded as when 

 they occur on young vigorously growing trees. It is easy to 

 perceive that, as a consequence, the prejudicial results of injuries 

 are increased as age advances. In this sense alone can we 

 speak of the feebleness of old age, and increasing susceptibility 

 to external dangers in consequence. 



The condition of vegetation in regard to the season of the year 

 has great influence on the power of the plant to resist dangers: 

 It is well known what low temperatures a plant can stand during 

 the period of winter rest, whereas in spring, after the beginning 

 of vegetative activity, and before it ceases in autumn, it is 

 killed by a few degrees of frost. 



The capacity of resisting the attacks of parasitic fungi on 

 the part of the tissues also differs much according to the 

 season of the year. Between the living cell of the host-plant 

 and the cell of the fungus- parasite there is a struggle, in which 

 (in the case of many parasites infesting the tissues of the cortex 

 or cambium) the latter can only kill the former if this is in 

 the condition of vegetative rest that is to say, not actively 

 growing, &c. If processes of metabolism are energetically at 

 work in the cellular tissue of the host-plant itself, it is then 

 enabled to stave off the attacks of the fungus. The action 

 of the latter on the cellular tissue of the host depending 

 as it does on the secretion of an enzyme * is only prejudicial 

 when this tissue is defenceless, as it were, owing to the condition 

 of inactivity in which it is found. These cortex-fungi grow 

 only from autumn till spring, and their further development 



* [Enzymes are a peculiar class of bodies, often called unorganised or 

 soluble ferments, capable of producing powerful molecular changes in 

 organic substances in presence of water. ED.] 



