1920] CURRENT LITERATURE 353 



3 feet of soil. This is true even of the deep rooted species, and is doubtless 

 to be related to distribution of soil moisture. It is notable that Markle 6 

 found a similarly abundant development of even more superficial laterals in 



| 



the very arid conditions in New Mexico. Markle also described and figured 

 a considerable variety of systems, and found rather definite layers of penetra- 

 tion lessening competition for the scarce water supply. 



In the succession from the gravel slide with coarse soil to the forest rich in 

 humus, the Colorado Rocky Mountains afford an interesting series. Weaver 

 finds decidedly superficial systems, both in the very sparsely populated gravel 

 and in the undergrowth of the forest, with the moisture distribution the con- 

 trolling factor in each case. The intermediate half gravel slide, with its sur- 

 face more than half occupied with plants, curiously enough has more deeply 

 rooted plants than the associations preceding or succeeding it. 



A comparison of species occurring in two or more different habitats shows 

 that of 10 species examined, 7 exhibit changes in root habit in response to the 

 changed environment, while 3 remain quite constant. Such studies of the 

 response of root systems to environment have attracted the attention of other 

 workers. Waterman 7 finds roots developing under dune conditions some- 

 what responsive to organic remains in the sand, although usually adhering 

 rigidly to their specific inherited form. Such rigidity was found by Pulling 8 

 in the shallow root systems of Picea mariana, Larix laricina, and Betula alba 

 papyri/era, as well as in the more deeply rooted Pinus Strobus and P. Banksiana; 

 while both the shallow rooted Picea canadensis and the deep rooted Populns 



batsamijera exhibited considerable plasticity. 



Cannon 9 believes that the roots of deeper penetration are less responsive 

 to changes in aeration and temperature than those of more superficial habit, 

 basing his conclusion upon the study of Pistacia atlantica and Prosopis 

 velutina of the former class, and Opuntia versicolor and 0. discata of the 

 latter class. The individuality of such responses is further shown by the 

 studies of Cannon and Free, 10 proving that while certain plants like Opuntia 

 stop root growth with a soil atmosphere of 50 to 75 per cent carbon dioxide, 

 others, like Prosopis, continue growth as long as 2 per cent of oxygen is 



6 Markle, M. S., Root systems of certain desert plants. Bot. Gaz. 64:i77~ 2 °5- 

 fi& s - 33- 1917- 



7 Waterman, W. G., Development of root systems under dune conditions. 

 Box. Gaz. 68:22-53. figs. 17. 1919. 



Pulling, H. E., Root habit and plant distribution in the far north. Plant 



World 21:223-233. fig. 1. 1918. 



9 Cannon, W. A., Modifications of root habits by experimental means. Carnegie 

 Inst. Wash. Yearbook 17:83-85. 1919. ' 



10 Cannon, W. A., and Free, E. E., The ecological significance of soil aeration. 

 Science N.S. 45:178-180. 1917. 



