THE PHYSICAL BASIS OF PARASITISM 239 



the summit of the tmnk. A cavity had been formed in the 

 trunk in the sinus formed by a branch, and this was heavily 

 Uned with callus and had a capacity of about a liter. The 

 tangled mass of roots of the prickly pear completely filled the 

 cavity holding together a compact mass of organic matter de- 

 rived partly from the original tissue now decayed. Near one 

 side of the root-mass, however, an active, strongly-growing 

 whitish root a few millimeters in diameter had pushed through 

 the callus walls and dowTi into a crevice of the live cells of the 

 Carnegiea. Its branches were spread out and were apphed to 

 the pale tissues of the host although no mechanical adhesion 

 of any consequence was detected. It was clear however that 

 some degree of absorption would be possible, and the appear- 

 ance of rapid growth on the part of the Opuntia was one which 

 could result only from a supply of material. 



The case was one of undoubted parasitivsm, and moreover the 

 dependent in this case had been in this position for an extended 

 period since the clump of roots was heavy and seven main 

 stems of the cylindrical form characteristic of the basal part of 

 the plant were found, each of which had at times produced 

 flattened joints. Some were now in a shrivelled condition and 

 doubtless many had been formed in the previous years for which 

 an adequate supply of solution was not available and they had 

 been cast off. 



Dr. W. B. MacCallum has recently discovered a similar case 

 of parasitism of an Opuntia on a sahuaro on the lands of the 

 company engaged in guayule culture at Continental, 30 miles 

 south of Tucson. An Opuntia with many joints was established 

 in the angle formed by a branch and the main stem. As the 

 parasite was not more than 2 meters from the ground it could be 

 readily photographed and examined. It was not disturbed, how- 

 ever. All of the tree cacti here had been bored by the "car- 

 pinteros" or woodpeckers, and it is probable that the seed from 

 which this plant originated had been deposited in a cavity made 

 by these birds (see fig. 1). The sahuaro, which had an age of 

 nearly two centuries, stood in the rectangular mass of stones 

 supposedly marking a prehistoric human grave. 



