44 THE ECOLOGICAL RELATIONS OF ROOTS. 



generalized root system is meant one that has both the tap and the 

 laterals well developed. They penetrate deeply and reach out widely. 

 In contrast, the speciaUzed root system has either the tap-root as the 

 chief feature or the laterals are placed near the surface and are espe- 

 cially well developed, as in cacti. The generalized type of root is much 

 more plastic and consequently reacts to a wider range of conditions 

 than does the specialized type. 



In considering the question of the susceptibihty of roots to modi- 

 fication through variation in the soil texture or its water-content, as 

 against the conservative inherited tendencies, table 13 is instructive. 



It appears that the root lengths on northeast and southwest slopes 

 are about the same and that the marked environmental differences 

 play Uttle part in determining root depth. The greater root depth of 

 most species on the northwest slope at a station which combined a 

 porous moist soil with rather extreme xerophytic above-ground con- 

 ditions is at once evident. In all habitats studied the writer has found 

 that hard soil profoundly affects the amount of branching, laterals 

 practically always being more numerous in a less compact substratum. 

 This may be closely connected with water-content and aeration — 

 the looser soils, of course, if of the same type, having the greater water- 

 holding capacity. A discussion of the effects of various habitats on 

 the root development of polydemics will be found on page 110. 



The roots of several species showed a marked increase in their out- 

 put of branches upon leaving the compact soil and entering earth- 

 worm burrows. In practically all cases the root-tips under such con- 

 ditions were alive and at least well covered with root-hairs, while in 

 the more compact soils of dry slopes especially the root-tips were 

 often dead and decayed. The cause of these differences is yet to be 

 determined. The differences may be due to the mechanical resistance 

 offered by the soils, or to changed conditions of aeration, or perhaps 

 to both factors acting together. Cannon and Free (1917) have found 

 that roots of various plants respond quite differently to variations 

 in the composition of the atmosphere. This difference appears to be 

 related to the character of the natural habitat of the species in ques- 

 tion. Their results indicate that plants growing in well-drained soil 

 are much more sensitive to the composition of the soil atmosphere 

 than those from poorly drained and poorly aerated habitats. 



Contrasting grassland species of the east and west, we reach the 

 following conclusions: Prairie plants of eastern Nebraska growing in 

 the clay-loam or loess soil under an annual precipitation of 28 to 30 

 inches, most of which occurs during the growing season, and with an 

 average daily summer evaporation of 21 c.c, have root systems very 

 similar in distribution but somewhat greater in depth than prairie 

 species of southeastern Washington growing in the silt-loam of dis- 



