DISEASES DUE TO SOIL-INFlUENCE 271 



and that have but poorly developed crowns in consequence, are 

 isolated by the removal of the beeches, they clothe their stems 

 abundantly with epicormic branches. For some years these as 

 well as the crowns thrive perfectly satisfactorily. In the process 

 of time, however and especially on the lighter classes of soil 

 which are subject to rapid drought or are liable to produce weeds 

 a portion of the topmost branches of the crowns die, and the 

 oaks become stag-headed. If the ground is protected in time, by 

 under-planting, the top branches either do not die or the disease 

 soon fails to make any progress, and the stag-headed condition 

 may entirely disappear owing to the dry branches dropping off. 



It is difficult to demonstrate the causes of these phenomena 

 experimentally, but the following explanation may be accepted 

 as sufficiently accounting for the disease. Directly the oaks are 

 isolated the amount of soluble food-materials in the soil is 

 augmented, owing to the accelerated decomposition of the humus 

 that covers the ground, and, at the same time, the leaves of the 

 crown, being more exposed to direct sunlight, are enabled to 

 assimilate more rapidly. These two causes combine to produce 

 considerable increase of the plastic substances, and consequently 

 an increase in growth, and the dormant axillary buds are also 

 enabled to develop into shoots. 



The first impulse to activity is probably communicated to the 

 dormant eyes by the increase in the products of metabolism, 

 while their further development into shoots is rendered possible 

 by the intensified action of the light. When the crowns and 

 branches have grown vigorously for a few years, the stock of 

 humus becomes exhausted, while the soil dries up in summer to 

 considerable depths, owing to the upper layers being deprived of 

 their protective covering. The result is that the processes by 

 which plant-food is rendered available are interfered with, and the 

 stock of soluble food-materials in the soil is reduced. Such a 

 state of things is commonly expressed by saying that the ground 

 has " become wild." 



The years when plant-food is abundant are followed by a 

 period of famine. Owing to the reduced supply of water and 

 nutriment, the upper part of the crowns is starved, the lower 

 branches appropriating the whole of the water and plant-food. 



Provided the crowns have not been too severely crippled, they 



