ROOT-SYSTEMS OF PERENNIALS. 69 



The root-system of the seedhng has a pronounced tap root with relatively 

 few and short laterals which do not bear haustoria if the plant is growing 

 alone, but develop them if the roots of another form, such as Parkinsonia 

 acnleata, are in close relation with its own. The root-system of the seed- 

 ling, however, differs from that of autotrophic plants in the fact that it does 

 not form root-hairs. As the plant develops, the tap root continues to be a 

 leading feature, since in the old plants remains of the tap root have always 

 been encountered. When or why this organ dies is not known. In the 

 mature plant the superficial laterals become the most prominent feature of 

 the root-system and include a large area within their compass. 



In associating character of root-system with habit in any species, it has 

 repeatedly been shown above that an autotrophic plant without water 

 reserve is always provided with a deeply placed root-system, or as deeply 

 as the soil permits, but that a plant with water-storage facility has a root- 

 system which lies near the surface of the soil. In Krameria the roots are 

 shallow, although the form is not a water-storing species — suggestive of 

 unusual life-habit of the species. 



Opuntia Arbuscula. 



Among the species of cacti limited to or characteristic of the bajada are 

 Opuntia arbuscula and 0. fulgida, though the latter only occurs on the 

 domain of the Desert I^aboratory The habit of the two arborescent cylin- 

 dro-opuntias is unlike. The species arbuscula occurs in groups of few 

 individuals usually, of common descent, as will be shown, while /w/^ifia at its 

 best forms fairly dense growths, to the exclusion of other woody plants. 



The specimen of Opuntia arbuscula chiefly studied was one of a small 

 group growing on the bajada about a mile east of the vSantaCruz river, April, 

 1907. The upper soil is of adobe clay to a thickness approximating 20 cm. 

 with a layer of rotten caliche beneath, of variable thickness, which rests on 

 the hardpan. 



In the vicinity of the cactus were Acacia constricta, Bigelowia hartwegii, 

 Covillca tridentata, Krameria canescens, Opuntia fulgida, and 0. spinosior. 

 Of these species, 3 plants of Covillea and one of Bigeloivia occurred within 

 the root-area of the cactus. The Covilleas were 20 cm. west, 70 cm. south, 

 and 2 meters northeast, all mature shrubs of large size. 



The Opuntia whose roots were examined was a much branched and old 

 plant about a meter high, apparently in perfect health and in every way a 

 normal specimen. Its root-system was composed of an anchoring portion 

 and a widely reaching horizontal portion (fig. 11). 



The anchoring roots were not specially studied. The horizontal system 

 consisted of 4 main members arising from the crown of the tap root just 

 below the surface of the ground. They radiated from the main axis in such 

 fashion that the ground around the base of the plant was fairly equally 

 divided between them. These main laterals, as distinguished from the cor- 



