28 THE ECOLOGICAL RELATIONS OF ROOTS. 



It should be noted here that these roses grew about midway up a southeast 

 slope. Here the loess soil was intermediate between that described for 

 Lygodesniia near the crest and for Amorpha near the foot of the hill. Beyond 

 a depth of 10 feet the soil was quite compact. 



Plate 15, B, illustrates well the paucity of large branches. The lateral 

 spread of any branch measured horizontally from the base of the crown did 

 not exceed 4 feet. Although many fine branchlets occurred along the course 

 of these main roots and extended off laterally for distances of 6 to 18 inches, 

 still other portions were quite free from branches. The breaking up of the 

 larger roots near their extremities into numerous long, slender, often more or 

 less parallel rootlets is well shown on the root in the figure, which ends at a 

 depth of about 6 feet. 



The older woody roots can easily be identified by scraping off the outer 

 black part of the thin cortex, which then reveals a bright red color, while 

 similar treatment of the younger roots shows their orange color. 



SHRUB ROOT SYSTEMS AND THE SHRUB ENVIRONMENT. 



From the preceding descriptions it may be seen that the members 

 of the chaparral community of the tension zone are all supplied with 

 splendid absorbing systems, which are somewhat variable as to depth, 

 but all of which are deep-seated. In addition, all have excellent 

 methods of vegetative propagation. 



The role played by these shrubs in modifying prairie conditions to 

 such an extent that the habitat becomes rather favorable to the growth 

 of the more xerophytic trees, such as bur oak, should not be overlooked. 

 All are well adapted, either by means of above-ground or under- 

 ground stems or root offshoots, to slowly but successfully invade the 

 prairie sod. For example, the whole loess hill, where these shrubs were 

 excavated, would probably be covered by a chaparral complex except 

 for repeated fires which are more detrimental to shrubs than to grasses 

 and most other prairie species. The proof of this statement lies not 

 only in the presence even over much of the more exposed hillside of a 

 potential scrub mictium, but also in the fact that Rhus, Symphori- 

 carpos, and other shrubs dominate over other portions of the same 

 ridge where soil conditions are identical but where fires have not been 

 permitted to run. 



Synphoricarpos extends its area by migrating by above-ground stems 

 as well as by those below the surface. This mechanism of invasion 

 is well illustrated in plate 13, b. While the above-ground stems furnish 

 the more rapid method of migration, they frequently fail to become 

 rooted in the prairie sod. They then become greatly attenuated and 

 soon die. The under-ground stems are more certain of establishment. 



Once established, this shrub rapidly reacts upon the habitat by 

 increasing the water-content both of air and soil, and also by modifying 

 the texture and composition of the latter. The presence of the shrubs 

 decreases wind movement and their shade reduces not only the light 

 but the temperature as well. This reacts favorably upon the humidity 



