ROnT'SYSTp:i\IS OF PEREXXIAI.S. 79 



The first plant studied was a separate individual with a shoot under 50 

 cm. in height, and extremely scragg}' and thorny. It possessed a stout tap 

 root which went down 75 cm. and then turning sharply ran for an unde- 

 termined distance horizontally. At its crown the root was elliptical in 

 section and 2 to 3 cm. in diameter. It gave off a single branch about 1 2 cm. 

 beneath the surface of the ground, and a single adventitious root was found 

 which took its origin at the crown of the tap root. This root was 5 mm. 

 in diameter and ran straight downward. Both the tap root and the adven- 

 titious root penetrated through the top soil, and the main root went to the 

 caliche before turning from its vertical course. (See plate 20.) 



The second study was made on a group of two plants of unequal size 

 and with indications that they had had a common origin. The tap root 

 of the larger plant went straight down, after the manner of the root of the 

 plant examined earlier, and bore two main laterals, of which one had been 

 partlv exposed bv the erosion of the soil surface, and one lay about 4 cm. 

 beneath the surface. The latter root kept its distance from the surface 

 fairly constant for the distance it was followed, i meter, and was seen to 

 give off frequent branches. The function of this root was evidently that of 

 absorption. The more superficial lateral was about 50 cm. long and along 

 its upper surface bore numerous small shoots, and on its lower surface many 

 short, slender rootlets. Xo deeply penetrating roots were found beside 

 the taj) root of the oldest, really parent, plant. It appears, therefore, in 

 these two plants, that the younger one depended on the parent for its con- 

 stant water supply, and that during the wet seasons this could be supple- 

 mented bv what was obtained bv small adventitious rootlets growing along 

 the lower surface of the superficial root. 



The third plant group studied consisted of two plants of unequal size, 

 but both relativelv small. The larger of the plants, the parent plant, had 

 a pronounced tap root which gave oft" a single prominent lateral on which 

 was borne, 15 cm. from its place of origin, the daughter plant. Midway 

 between the two plants the connecting root was observed to be dead and 

 partly decayed. An examination of the daughter jjlant showed that it had 

 several slender and short absorptive roots, but that in addition there was 

 one of larger diameter which ran straight down and functioned as a tap 

 root (plate 20). The daughter plant, springing as a shoot from the sucker- 

 like root of the larger form, had finally become independent and had devel- 

 oped a deeply penetrating root-system of its own. 



Other plants were examined in which the daughter shoots were large and 

 were borne on stout superficial laterals of the parent plant but which had 

 not developed tap roots. In the largest of the shoot-bearing laterals no 

 absorptive adventitious roots w^re seen, but such rootlets are present in 

 large numbers on such laterals as the one shown. 



It would appear from observations on the distribution and root habits 

 of Kccrbcrlinia . that it must be in continuous connection with a water 

 supply. This is eft'ected cither by each plant developing a tap root of its 



