General Considerations 199 



and that all species thrive best on one kind of soil (see page 

 24), but some can be grown under less favorable conditions; 

 enduring, however, not preferring, as some would have it, 

 drier or wetter situations and shallow soils. 



\\'hile, for the start of the tree, the soil in the plant hole can 

 be improved, it must not be forgotten that, in most cases, 

 tlie tree must eventually grow into the native soil, in which 

 it must be adapted to help itself. Especially is this true in 

 regard to the depth. There exist, however, wrong notions 

 as to the depth of soil needed: a soil of four to six feet is 

 deep for most species, and if fissured rock underlies the sur- 

 face soil at from two to three feet, the conditions are favor- 

 able enough for adaptive species, even with deep-going roots, 

 the roots penetrating into the fissures which form good 

 drainage channels. Only when impenetrable layers of rock 

 or ground-water lie within two or three feet from the surface 

 will species with tap-root or heart-root fail to tlirive, eventually 

 showing the effect in rapidly tapering and spindling form. 



In streets, the natural deficiencies of the soil are further 

 accentuated by such disturbances of water-supply as the 

 impediment of pavements, and the underground drain pipes, 

 etc., under which conditions hardiness and adaptiveness as 

 regards root development arc naturally most essential. 



We should here again lay stress upon the fact that there 

 is a more or less close interrelation between soil and chmate, 

 and that the one can in part compensate for the other; 

 that is to say, a poor soil (as regards water-supply) will 

 accentuate the deficiencies of climate, while a deep, well- 

 watered, well-drained soil will make it possible for species 

 to endure cHmatic ills to which they would succumb under 

 less satisfactory soil conditions. Hence hardiness is, at 

 least in part, dependent on soil. 



Similarly, there may be provided some natural or artificial 



