THE BEGINNINGS AND PHYSICAL BASIS OF 



PARASITISM 



D. T. MACDOUGAL 



Desert Laboratory of the Carnegie Institution, Tucson, Arizona 



A series of experiments for the purpose of determining the 

 major conditions under which one seed-plant, ordinarily auto- 

 phytic, might become parasitic upon another were begun at the 

 Desert Laboratory in 1908. The method of making the prepara- 

 tions, the behavior of the xeno-parasites, and the absorbent 

 relations of the enforced host and experimental parasite have 

 been described by the author in various papers. ^ 



In addition to the experimental tests several cases have been 

 encountered in the field work from the Desert Laboratory in 

 which Opuntias were seen established on Parkinsonia, Acacia 

 and Carnegiea -wdth the roots in nutritive contact with the 

 tissues of the supporting plant and under such conditions that 

 the sole supply of solution must have been received from the 

 newly-found host. 



An instance of this kind was encountered on March 15, 1913 

 when a field party found a sahuaro 57 miles southwest of the 

 Desert Laboratory bearing near its summit an Opuntia, which 

 was in all certainty Opuntia Blakeana, (probably equivalent to 

 0. phaeacantha) . During the course of cutting down the tree 

 cactus for an examination of the relations of the two plants, 

 a Papago Indian came up and showed the author the same 

 species in the soil nearby, and he gave the native name as 

 "e'epa." It is identical with the Opuntia which has been found 

 on Carnegiea m another instance. 



The tree cactus was about 20 feet in height and bore two 

 large lateral branches, while a third had been broken off near 



1 See MacDougal, D. T., Induced and Occasional Parasitism. Bull. Torr. 

 Bot. Club. 38:471. 1911. 



An Attempted Analysis of Parasitism. Bot. Gaz. 52, 249, 1911. 



238 



