PHVSIOLOGICAU FEATURES OF ROOT-SYSTEMS. ()l 



mer annual, outlives many other forms, and should additional rains occur 

 somewhat out of season, but before the plant has succumbed, it will revive 

 and accompHsh an astonishing extent of growth. Among the winter 

 annuals, such deeply rooted forms as Amsinckia spcctabilis and Phacdia 

 tanacetifolia will survive long after Bowlcsia lobata, or other plants with 

 shallow roots, have perished. 



How closely the character of the water supply, the amount of water, 

 and the progressive drying out of the soil, are related to the character of 

 the root-system of annuals, does not appear from the observations here 

 reported. In certain instances, however, the root-system is greatly modi- 

 fied by the water relations. For example, it has been repeatedly seen that 

 the longest and frequently the greatest number of laterals arise within 4 to 5 

 cm. of the surface of the soil. Without definite knowledge of the water 

 movements at that depth, it may be assumed that during the period 

 of greatest growth the soil at this depth has an adequate water supply 

 and provides practically all of the water necessary for the plant, and that it 

 is only later, when by progressive drying out the upper soil becomes too 

 dry for benefiting the plant, that it obtains its water at the greatest depth 

 and mainly by means of the elongated tap root. In certain cases, as in 

 Amsinckia, where precociously formed root rudiments are present, the 

 coming of unseasonal rains induces growth, and the superficial roots are 

 again in condition to absorb water. 



The reaction of the root-systems of the perennials is much more complex 

 than that of the annuals, inasmuch as a portion of the roots live through- 

 out the year and a portion endure during favorable seasons only. The situ- 

 ation is further complicated by the fact that each habitat differs from every 

 other habitat as regards water relations. The great difference in the char- 

 acter of the root-systems of the perennials and their possible variability 

 under different conditions have also to be considered. 



The peripheral rootlets of the generalized root-systems are probably 

 living throughout the year, since they are in soil which, even in dry seasons, 

 contains sufficient moisture for absorption, but the deciduous rootlets, both 

 of the generalized and the specialized root-systems, are present and func- 

 tional only during the more favorable seasons, -chiefly in summer. The 

 peripheral roots of the specialized forms of the cactus type, however, like 

 the deciduous rootlets, are present only during the more favorable seasons. 

 From these circumstances it is seen that the active period of absorption of 

 plants with the latter type of roots is a very restricted one, while the absorp- 

 tion period of the generalized root-system and of the Ephedra type of the 

 specialized system, although most active during the existence of the decid- 

 uous rootlets, is continuous from season to season. 



In plants having generalized root-systems, it is probable that the pene- 

 tration of the roots, the character of the soil permitting, is equal to the 

 penetration of the rains. On the bajada this is limited by the presence of 



