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210 The Plant World. 



use of massive plants also rendered it easy to make the chemicaj 

 analyses necessary to determine the concentration of the cell- 

 sap, which was thought to be of importance in this connection. 

 The most direct method of ascertaining the relative osmotic value 

 of the sap of two plants would be by freezing tests of the ex- 

 pressed juice. 



The native flora around the Desert Laboratory at Tucson. 

 Arizona, is rich in succulents which furnish the conditions noted 

 above, and the material chosen for experimentation in the present 

 work included chiefly several opuntias which were used as both 

 host and parasite, normal and in etiolated condition, Carnegiea 

 gigantea, exclusively as host, Echinocactus wislizeni as host. 

 Fouquieria splendens as both host and parasite, Cissus digitata 

 and C. lacinata from southern Mexico, Agave americana, Cot) 

 ledon macrantha, and Tradescantia as parasites, in addition to 

 a number of other forms of which but slight use was made. After 

 some preliminary tests, the arrangement of the material in the 

 dependent relation, was begun seriously in January, 1908, and 

 many hundreds of preparations have since been set up. 



The methods of experimentation employed consisted in 

 making cuttings of the joints of prickly pears, Mexican grape 

 vine, agave and of other plants which were to be induced to live 

 parasitically and thrusting these into a bed of sand until the raw 

 surfaces were healed and regeneration with root formation had 

 begun. Next, a suitable cavity was cleanly cut in the flesh of a 

 giant cactus, or barrel cactus, or in the plant to be Used as a host, 

 into which the base of the slip of " xeno-parasite " was thrust. 

 The insertion was held in place by an adequate setting of plaster 

 of Paris, and the whole preparation was cared for as to shade and' 

 temperatures. If the inserted slip was capable of immediate 

 adjustment to the new conditions, interesting results followed. 

 The transfer of a plant from its original state to the experimental 

 arrangement described, however, is an extremely radical one, 

 and involves serious disturbances of a group of functions. 



Thus, in the insertion of slips of one plant in thebody of another, 

 the development of the roots would necessarily be carried on in 

 confined cavities, in which the only free oxygen available from 

 the growth of these organs would be that coming from the inter- 

 cellular spaces of the host and by transfusion. The lack of this 



