26 



THE ECOLOGICAL RELATIONS OF ROOTS. 



in length from those only an inch long to others extending a distance of 2 or 

 more feet, sometimes almost at right angles. The last foot or 18 inches of the 

 root is usually threadlike but extremely well branched. Above the level of 

 65 inches the soil is well filled by great numbefs of these fine root-endings. 



Rhus glabra. — trench 10 feet long was dug just within a thicket of sumac 

 which, according to ring counts, had been in possession of the area for at least 

 12 years. Practically all herbaceous species had disappeared except Poa 

 pratensis. A brief examination showed the soil to be filled with a network of 

 rhizomes from which numerous roots ascended more or less vertically and 

 ended at the surface of the soil in a brush-like mass of branches. Further 

 study revealed the really wonderful complexity of these underground parts 

 as well as the great absorbing area of this xero-mesophytic shrub (plate 

 14, A, b). Roots were examined in three different situations. Although they 

 differ in details, the following is a tjrpical example : 



A tap-root 3 cm. in diameter and 12 or 13 years old pursued a downward 

 course at an angle of about 40° from the vertical to a depth of 22 inches. Here 

 it turned off at an angle of 30° from the horizontal and ran a distance of 2 

 feet, at which length (48 inches) it was only 1.5 cm. in diameter and 37 inches 

 deep. Here it forked into two equal branches, one (a) running almost hori- 

 zontally to a distance of 68 inches and at a depth of 3 feet, while the other (b) 

 turned off at an angle of 40° from the first and, after running rather obliquely 

 downward for a distance of 15 inches, gave off two horizontal laterals 3 and 6 

 mm. in diameter respectively, while the main root turned vertically upward. 

 It ran in this direction for a distance of 8 inches, when it divided into 3 equal 

 branches. Two of these were each 6 mm. in diameter and tapered gradually 

 till they reached the surface, 26 inches above, and at a horizontal distance of 

 26 inches beyond the vertical root from which they branched. They ended 

 about 2 feet apart in an extremely well-branched and rebranched network 

 of fine rootlets. These root termini were 70 inches in a horizontal line from 

 the beginning of the tap. 



The third branch from the upright portion of the root turned off horizon- 

 tally and ran a distance of 8 inches, when it again turned at right angles and 

 ran horizontally in another direction (away from the main tap) 16 inches, 

 finally turning downward at right angles and descending to a depth of 80 

 inches. The tip, which ended in a much-branched network of almost hair- 

 like terminals, was located at a horizontal distance of 68 inches from the 

 beginning of the tap. The two 3 and 6 mm. laterals of root (6) ran off hori- 

 zontally, but in a direction opposite from the others, finally ascending to the 

 surface several feet from the other roots. Branch (a), which was partly cut 

 off in digging the trench, also sent off mmaerous roots to the surface as well as 

 some into the deeper soil. The surface soil is filled with rootlets with such a 

 network of absorbing terminals that competition with the grasses for soil 

 moisture must be very severe. Nor is the main absorbing area confined to the 

 surface. Well-branched laterals, both large and small, arise at all depths, and 

 even the deeper soil is rather thoroughly occupied. The well-branched root 

 system, together with the large area it occupies, must afford abundant mois- 

 ture, even in fairly dry soil. 



While all of the main roots examined in two separate thickets showed the 

 obhque to horizontal course with a depth never exceeding 80 inches, one root 

 of a Rhus plant growing in the Symphoricarpos thicket was 0.5 inch in diam- 

 eter and pursued a vertically downward course to a depth of over 90 inches. 

 The roots are characterized by a thick cortex which exudes a white latex when 

 injured, and upon removal reveals the small, woody, glistening white stele. 

 The younger roots vary in color from brown to nearly black. 



