112 STUDIES OF TKEES 



roots of the tree from the soil. The liquid absorbed by the 

 roots carries in solution the mineral salts the food of the 

 tree and no food can be taken up unless it is in solution. 

 Much of the water is used by the tree and an enormous 

 amount is given off in the process of evaporation. 



These facts will explain some of the fundamental prin- 

 ciples in the care of trees. To a tree growing on a city 

 street or on a lawn where nature fails to supply the requisite 

 amount of water, the latter must be supplied artificially, 

 especially during the hot summer months, or else dead 

 branches may result as seen in Fig. 89. Too much thinning 

 out of the crown causes excessive evaporation, and too much 

 cutting out in woodlands causes the soil to dry and the trees 

 to suffer for the want of moisture. This also explains why 

 it is essential, in wooded areas, to retain on the ground 

 the fallen .leaves. In decomposing and mixing with the 

 soil, the fallen leaves not only supply the trees with food 

 material, but also tend to conserve moisture in the ground 

 and to prevent the drying out cf the soil. Raking off the 

 leaves from wooded areas, a practice common in parks 

 and on private estates hurts the trees seriously. Some 

 soils may have plenty of moisture, but may also be so 

 heavily saturated with acids or salts that the tree cannot 

 utilize the moisture, and it suffers from drought just the 

 same as if there had been no moisture at all in the soil. 

 Such soils are said to be " physiologically dry " and need 

 treatment. 



In the development of disease, moisture is a contrib- 

 uting factor and, therefore, in cavities or underneath band- 

 ages where there is likely to be an accumulation of moisture, 

 decay will do more damage than in places that are dry 

 and exposed to the sun. 



(2) Influence of soil: Soil is made up of fine particles 



