COVILLEA TRIDENTATA. 11 



into the relations of parasite and host, for which reason a fuller account 

 is omitted at this time. 



COVILLEA TRIDENTATA. 



In the vicinity of the Desert Laboratory Covillea, as regards habitat choice, 

 exhibits great catholicity of taste. It not only occurs on the bajadas, where 

 it is the characteristic species, but also on steep mountain-slopes and on 

 the flood-plain. This means, as will appear directly, that it grows in both 

 deep and shallow soils and where there is perennial water as well as where 

 water is only abundant during the rainy seasons. Along with this wide dis- 

 tribution the species has an extremely plastic root-system by which in the 

 diverse habitats the needs of the plant are well accommodated. The par- 

 asitic relationship with Krameria was demonstrated in specimens of Covil- 

 lea, both from the bajadas and from the alluvial flat west of Tumamoe Hill. 

 The soil of the flat reaches to a depth of over 2 meters and offers a very 

 favorable opportunity for deep root -penetration. The root-system of a 

 specimen of Covillea growing in this location had the following character- 

 istics: The tap-root extended directly downwards nearly 2 meters and gave 

 off numerous branches, for the most part near the surface of the soil. In 

 addition to these laterals, others also rose from the bases of the shoots. 

 After leaving the central axis the laterals ran for the most part from 20 

 to 45 cm. beneath the surface of the ground, and some of them gave off 

 branches which ran directly downward as deep as the tap-root. The lat- 

 erals did not reach out from the main body of the plant for a distance 

 exceeding 2.2 meters. Only the most superficial of the roots of Covillea 

 were in position to be attacked by those of Krameria. 



The soil conditions of the plant from the slopes or bajadas were very 

 different from those for the specimen from the fiat. On the slopes the 

 upper and most permeable soil, the adobe, is only about 20 cm. in thickness. 

 This is underlaid by a stratum of the same thickness which is part 

 adobe and part caliche, and, for convenience, is referred to as "rotten 

 caliche." Beneath the rotten caliche is hard or solid caliche of indef- 

 inite depth. This is practically impervious to water, except in rifts or 

 seams, where adobe takes its place. 



The specimen of Covillea from the bajada which was especially studied 

 had a main root which went down to the hard caliche, where it forked, one 

 branch going about 3 meters along the upper surface of the caliche, the 

 other running for a considerable but undetermined distance on the caliche's 

 surface. The greatest depths attained by the branches were 45 and 53 

 cm. in a place where there was a depression in the caliche. These are 

 the most deeply placed roots of the plant. Nearly a dozen large laterals 

 and numerous small ones were given off from the main root within 15 cm. 

 of the surface of the ground, usually nearer than that, and the larger took 

 a more or less direct and fairly horizontal course for a distance of about 



