50 



THE ECOLOGICAL RELATIONS OF ROOTS. 



1.5 mm. in diameter and 27 inches long. A similar lateral about the same 

 size arose 6 inches deeper in the soil. At 5 feet the tap, now only 1.5 mm. in 

 diameter, forked dichotomously. At 6 feet the two branches entered a layer 

 of moist sand through which they ran for 26 inches to a total depth of 8 feet, 

 but they gave off very few branches. Rootlets of other plants of this species 

 were quite numerous in these deeper soils. Finally, a plant was excavated 

 and photographed (plate 18, b) which reached a depth of 12 feet 2 inches. 

 The roots are brown to reddish-brown in color. 



Artemisia frigida. — Mountain sage is a competitor of the plains grasses, 

 especially northward, for the rather meager water-supply. Near the moun- 

 tains, in rocky or gravelly situations, it frequently forms extensive societies. 



From the base of the clustered woody stems a tap-root arises; it is from 5 

 to about 10 mm. in diameter and descends rather vertically to a distance of 

 from 4 to over 6 feet. The tap, however, becomes rapidly attenuated, so that 

 at a depth of 6 inches it is scarcely 1 to 2 mm. in diameter and indeed no 

 larger than the numerous strong, rather horizontal laterals which arise from 

 the tap or the base of the woody stem at just below the ground surface. These 

 larger laterals, frequently as many as 5 to 9 on a single plant, run off obliquely 

 in the shallow soil, sometimes ahnost horizontally and from a depth of only 

 1 to 2 inches to a distance of 8 to 12 inches or more before turning rather 

 abruptly downward. The woody bases of the stem, as well as the first 6 to 10 

 inches of tap-root, give rise to very abundant rootlets, 1 nam. or less in diameter, 

 which thoroughly fill the surface soil, branching and rebranching into thread- 

 like termini and furnishing Artemisia, as is the case with so many plains plants, 

 with a splendid surface absorbing system. 



Like the tap-root, the larger laterals upon turning downward become 

 greatly attenuated, the diameter remaining imiformly a millimeter or less for 

 several feet. When near the ends they often become hairlike but are not well 

 branched. The deeper roots, while branching from time to time and pursuing 

 a rather tortuous course through the soil, are characterized by the absence of 

 numerous laterals. A large number of plants were examined and most of the 

 major branches extended with the tap to depths of from 3 to 6 feet. The 

 deepest root examined entered at a depth of 5 feet into the soft soil filling an 

 ancient burrow, in which it continued, giving rise from time to time to long, 

 threadlike, unbranched laterals. It reached a depth of 7 feet 9 inches. The 

 roots are chestnut-brown in color and all but the larger ones are very brittle, 

 being removed with extreme difficulty from the hard, lumpy soil (fig. 10). 



Argemone platyceras. — This wild poppy, conspicuous because of its large 

 size and showy white flowers, is found as a frequent component of plains 

 vegetation throughout much of the association. As a rule, it is more abundant 

 in disturbed areas, often becoming ruderal. 



The plant examined had a strong tap-root. It was partially decayed, por- 

 tions of it being dead and easily dug out. At the end of the first foot, however, 

 it branched dichotomously. These branches diverged not more than 6 to 8 

 inches and took a downward course, continuing to branch dichotomously at 

 irregular intervals for the first 8 feet, the whole trend being downward (fig. 

 13). At no point in the first 8 feet of soil did the branches spread more than 

 1.5 feet from the vertical. At about 8 feet a layer of moist sand was encoun- 

 tered and here some of the branches turned outward almost horizontally, 

 extending 18 to 24 inches, where they ended. Other branches continued 

 downward through moist sand to a maximum depth of over 12 feet, the last 

 10 to 12 inches of their length passing into a very moist sandy clay. From 



