PLANT PHYSIOLOGY 27 



results described by Barker and Spinks {Long Ashton Agric. and 

 Hort. Res. St. Ann. Rep., 191 7, 43-54), who worked out a general 

 practical classification of the free apple stocks on the basis 

 of root characteristics, which varied from an entirely fibrous 

 system to a tap root without fibres. These authors conclude 

 that it is not possible entirely to correlate differences of vigour 

 with these root differences. J. E. Weaver {Carnegie Inst. 

 Wash. Publ., 286, 191 9), in a comprehensive study of the root 

 systems of plants in their natural habitats, made excavations 

 and examinations of roots of different families, habits, and 

 habitats and found some striking differences. In one habitat, 

 for example, whilst the grasses were rooted generally in the 

 upper two feet of soil, the majority of the roots of dicotyledons 

 were found at depths between two feet and twenty feet. This 

 is remarkable in view of the fact that there is frequently no 

 soil moisture available below the five-feet level in the regions 

 concerned. Contrary to the conclusions of many observers, 

 Weaver considers that, except for a few very stable types, most 

 plants are able to conform their root systems to the habit of 

 the community of which they form a part. He agrees with 

 Markle that soil moisture is a determining factor in the distri- 

 bution of roots, in spite of his observations on the depth of 

 root penetration and the availability of water. Soil moisture 

 is considered by Waterman {Bot. Gaz., 1919, 68, 22-53) to be 

 less important than other factors in causing root variation, 

 and this in a dune habitat, where the competition for water is 

 likely to be severe. This writer finds that the response of 

 roots to the peculiar soil conditions varies with different species. 

 He considers that the main variations are attributable to 

 nutrition rather than to the physical characteristics of the 

 situation, such as penetrability of the soil. The extreme 

 variation of the proportion of shoot to root is cited as a warning 

 against the measurement of plant growth by estimations of 

 root production. Other ecological facts with physiological as- 

 pect are recorded by Pulling {Plant World, 191 8, 21, 223-233), 

 who has investigated the adaptability of root habit in the 

 northern latitudes of America. He shows that a tree with a 

 rigid, immutable, deep root system is automatically excluded 

 from shallow soils. This factor in conjunction with increasing 

 shallowness of thawed soil, probably determines the northern 

 limit of Canadian forest trees, irrespective of climatic environ- 

 ment. All types of root system were found to exist, from 

 tap-root systems to fibrous superficial systems, with varying 

 capacity for adaptation. 



Coupin {Compt. Rend., 1919, 168, 1005-1008) contributes a 

 paper on root absorption. He finds that the root tip is the most 

 important region for absorptive purposes, and that no advan- 



