74 



THE ECOLOGICAL RELATIONS OF ROOTS. 



from the base of the stem, running more or less parallel with the soil surface 

 at a depth of 2 to 5 inches and to a maximum distance of 30 inches. 



All the roots, whether superficial or deeper, were practically free from 

 branches for a distance of 4 to 6 inches. From that point on they began to 

 branch, the branches increasing in number toward the tips. These branches 

 varied from a few millimeters to 3 or 4 cm. in length, and at intervals on these 



Fig. 26. — Tradescanlia virginiana. 



were found short hairlike branches 2 to 5 mm. in length. While the root 

 system of this plant was fairly well branched, the branches were in the main 

 not nearly so numerous as those of most of the other sandhill perennials. The 

 roots were light brown to white in color, fleshy and brittle. Six plants were 

 examined. 



Heliotropium convolvulaceum. — This annual frequently forms small fami- 

 lies or colonies on the rims of blowouts and on half-captured sandy slopes. The 

 plants were examined on August 1. They were quite mature and in full 

 bloom. None was over 7 inches high. They are characterized by a small 

 tap-root, seldom over 2 or 3 mm. in diameter, which maintains its dominance 

 in spite of the abundant laterals and reaches depths of 18 to 25 inches. 

 Throughout its rather vertically downward course, after passing through the 

 surface inch of dry sand, it continually gives off laterals, both short and long, 

 to the very tip. While many of these exceedingly fine branches do not exceed 

 a few centimeters in length, others with a diameter of 0.3 to 0.5 mm. extend 

 for a distance of 6 to 20 inches, often in a rather horizontal direction with the 

 soil surface. As a whole they are poorly supplied with smaller rootlets. Some- 

 times these run out obliquely for a distance and then turn downward. A 

 single root may have 4 to 6 of these long laterals. They branch at intervals 

 into long threadlike branches, which are themselves only moderately well 

 supplied with absorbing rootlets. However, the lack of abundant branches, 

 such as occur in many grasses and dicotyledons, is offset by the delicacy of the 

 root system, all parts of which are efficient absorbers. These herbaceous 

 roots are rather tough; they are light tan in color. 



Petalostemon villosus. — This low, finely branched plant with its dense cover 

 of silvery leaflets stands out as a prominent object in the sandhill flora, where 

 it regularly plays the role of a pioneer in the blowouts. Sometimes it occupies 

 such situations to the complete exclusion of other species. Of the half-dozen 

 mature plants examined, all had strong tap-roots, about 1 cm. in diameter, a 

 much-branched root system, and a root penetration of 4 or 5 feet. Figure 27 

 is illustrative of these. An examination of this figure reveals the numerous 



