68 



ROOT HABITS OF DESERT PLANTS. 



Fig. 10. — Root-system of Krameria canescens. 

 The large laterals d, d, d were dead. 



feature of the root-system of the plant was the almost total absence of 

 branches, nor were the filamentous roots which occur in Franseria, Encelia, 

 and in other perennials, obser\^ed in Krameria. 



Only two foreign roots 

 (shown as dotted lines, fig. 

 lo) of an unknown species 

 were encountered during 

 the excavation of roots of 

 Kravieria. 



The presence of so many 

 dead roots made it advis- 

 able to examine another 

 specimen of equal age in 

 order that a perfectly "nor- 

 mal" root-system should 

 be described. Accordingly, 

 another plant, also growing 

 on the flood-plain, was 

 selected for study. The 

 results of the second excava- 

 tion substantiated those of 

 the first, namely, that the tap root had reached a rather large size before 

 dying, that there were many dead laterals, and that those yet living were 

 near the surface, although the soil at the place was of considerable depth. 

 In examining another plant, under circumstances which allowed the obser- 

 vation of the relation of the roots of the species to those of its neighbors, 

 the fact of the parasitic habit of the plant was revealed. 



After the discovery of parasitism in Krameria a very large number of 

 specimens growing under widely different conditions were examined and 

 root attachment to the following plants was demonstrated: Acacia con- 

 stricta, Covillea tridentata, Encelia farinosa, Ephedra trifurca, Franseria 

 deltoidea, Lyciwn andersonii, Menodora scabra, Opuntia sp., Parkins onia 

 microphylla, Prosopis velutina, and Zizyphus parryi. Subsequently para- 

 sitism of the plant on Parkinsonia aculeata, a species indigenous within 75 

 miles of Tucson, was induced in cultures. 



The attachment to the host is usually by means of small roots and only 

 rarely does one find the association of old roots of host and parasite. Thus 

 the connection is probably made during the growing seasons, and persists 

 if the conditions continue favorable during the subsequent dry periods. 



The hosts already found are only trees and shrubs, and it is not at present 

 known whether the parasite is strictly limited to such, or whether it may 

 not derive temporary sustenance from annuals, some of which are very 

 resistant to drought and endure for considerable time after the rainy seasons 

 have closed. 



