[4 DISEASES OF TREES 



the course of time, varieties have spontaneously arisen whose 

 power of resisting frost, or atmospheric drought, as the case may 

 be, has become enhanced. 



A further group of disease-inducing conditions embraces all 

 these peculiarities which have only been acquired in the course 

 of the development of the plant, and which may lead to a disease 

 if certain external influences be present. If plants are reared in a 

 moist atmosphere, e.g. in a greenhouse, the epidermal system 

 develops in response to the moist air which surrounds it, so that 

 it is only slightly cuticularized. If such plants are placed in a 

 dry atmosphere for instance, in the air of a heated room they 

 become sickly, because the transpiration of the leaves is unduly 

 increased. 



Trees, especially those with smooth periderm, that are reared 

 in a very dense wood, and then suddenly isolated in later life, 

 suffer from scorching of the cortex. Such trees possess a pre- 

 disposition for scorching which is absent in the case of those 

 plants of the same species which have been grown, from youth 

 upwards, in an open or light wood. The disposition to disease 

 in this case consists in the fact that the external covering is 

 less strongly developed. Plants grown in the shade also prove 

 to be unduly susceptible to the direct action of the sun, in 

 that the chlorophyll in the cells of the upper layers of their 

 leaves becomes destroyed. Oaks grown in a close beech wood, 

 and consequently with small crowns, incur a predisposition for 

 top-drought when they are isolated, whereas, under similar 

 circumstances, trees with full crowns do not suffer from this 

 disease. 



During the first few years after being transplanted, many 

 trees show a predisposition to be easily " frosted," which is again 

 lost with the development of a strong root-system. On shallow 

 soils evergreens, and especially conifers, are far more susceptible 

 to injury from coal-smoke than those on deep soils, for the 

 reason that their root-system, being characterised by superficial 

 development, is unable to take up water in winter. Desiccation 

 of the leaves in consequence of action of the sulphurous acid 

 takes place in their case much more easily than in that of trees 

 which are able to take up water from greater depths, and this 

 even in winter. 



