INTRODUCTION. 9 



laterals which are superficially placed, and Weiss and Yapp (Sketches of 

 vegetation at home and abroad, III. "The Karoo" in August. The New 

 Phytologist, vol. v. May and June, 1906) have reported the formation of 

 superficial in addition to deep-growing roots in Mesembrianthemiim. In a 

 brief account of the habits of several cacti of Arizona Preston also describes 

 the shallow placing of the roots of several of them (Botanical Gazette, vol. 

 30, p. 348, 1900). 



The dwarfing of the shoots of the perennials of the deserts, resulting from 

 excessive evaporation and inadequate water supply, does not indicate the 

 best conditions for maximum root development. It is probable, on the 

 contrary, that the longest or the most deeply penetrating roots are found 

 where there is considerable rainfall and where the penetration of the rain 

 is considerable and the water table relatively deep. In California, under 

 field conditions, the roots of the grape may reach a depth of 22 feet (Hilgard : 

 Soils, p. 167), and in Nebraska the roots of Shcpherdia are said to attain a 

 depth of 50 feet (Merrill: Rocks and Rockweathering, p. 181). 



CERTAIN FEATURES IN THE ATTEMPTED CLASSIFICATION OF ROOTS. 



From the point of view of this study any classification of root-systems 

 which has for its final aim a better understanding of the habits of plants 

 should receive attention, while those classifications which are purely sys- 

 tematic, although of value in other lines of research, may in this place be 

 neglected. Attention will be called, therefore, merely to the work of Rim- 

 bach, Biisgen, and Freidenfeldt, mainly as reviewed by von Alten (Wurzel- 

 studien, Bot. Zeit., vol. 67, 1909, p. 175), which is to be largely interpreted 

 in physiological terms. 



The researches of the authors referred to indicate that the root-systems 

 of flowering plants may be separated into two groups according to the 

 character of the terminal roots — they are either intensive or extensive. 

 Intensive root-systems are such as have fine terminal roots ; they are richly 

 branched and occupy a relatively small soil volume. Extensive root- 

 systems, on the other hand, are such as have coarse ultimate rootlets, are 

 not richly branched, and occupy a relatively large soil volume. An im- 

 portant additional distinction, advanced by Freidenfeldt, is that of the 

 diameter-quotient, or the relative diameter of the central cylinder of such 

 rootlets to that of the entire rootlet. Von Alten states that the diameter- 

 quotient in intensive root-systems is greater than in extensive root-systems. 

 For example, the difference may be from one-third or one-fourth to one- 

 fifth or one-sixth, respectively, for the two types. In addition, there are 

 structural differences which need not be given in this place. 



Freidenfeldt, in place of the term intensive, classifies plants having these 

 root types as xerophytes, and in place of the term extensive, classifies 

 plants with this type of root-system as hydrophytes. The beech is given 

 as an example of the former and the ash as an example of the latter, but 



