272 DISEASES OF TREES 



may recover under the influence of an increased supply of food- 

 materials, consequent on the improvement of the soil by under- 

 planting. Trees that were possessed of well-developed crowns 

 before the wood was light-thinned produce but few, if any, 

 epicormic branches, nor do they become stag-headed. The 

 reason for this is that, owing to vigorous development, the 

 crowns are able to make use of the excess of nutriment that 

 is produced during the years immediately following the light- 

 thinning. No epicormic shoots are produced, so that there are 

 none to interfere with the nourishment of the crowns during the 

 years of famine. No doubt the general health of the crowns 

 suffers, but at all events their upper branches do not die. 



It follows from what has been said that if top-drought is to be 

 avoided there must be no temporary reduction of soil -fertility. 

 The discovery of the means by which the soil may be protected 

 and its fertility conserved falls within the province of sylviculture. 



In the case of agricultural plants we are familiar with a 

 number of pathological phenomena which are primarily due to 

 the effects of drought on the soil. Here I will only mention 

 the " going off " of cereals namely, the withering of the straw 

 before the grain has formed and the premature ripening of grain, 

 where the plants wither after the seeds have formed, but before 

 all the nutritive materials have been stored up in the grain. 



Under exceptional circumstances it may also happen that the 

 growth of plants is interfered with by excess of nutriment. I 

 would, however, again utter a word of warning against hastily 

 ascribing sickly appearances to the soil, in the absence of 

 scientific evidence. A sudden increase in the supply of plant-food, 

 and the consequent important augmentation of the plastic sub- 

 stances, may, under certain circumstances, cause the outer tissues 

 to rupture, and this occurs when their extension has been unable to 

 keep pace with the growth of the internal tissues. It occasionally 

 happens, when some cultural operation has suddenly induced 

 considerable increase of growth in trees, that the bark, especially 

 on the main stem, is ruptured on all sides owing to the powerful 

 internal pressure that is set up. When hornbeams 1 that were 

 mixed with beeches in a wood were suddenly isolated in the 

 seed-felling, their annual sectional growth at breast-height 

 1 Untersuchungen aus dem Forstbot. Inst., Vol. III. pp. 141 144. 



