Igtt] MACDOUGAL—PARASITISM 251 
In the experiments carried on at the Desert Laboratory from 
1908 to 1911, the desert grape (Cissus laciniata) of Mexico, the 
expressed juice of which shows a pressure of over 11 atmospheres, 
was found to maintain itself on the flattened joints of Opuntia 
Biakeana at about 9 atmospheres; not so successfully on Echino- 
cactus Wislizeni, the drinkable juice of which has a concentration 
equivalent to about 6 atmospheres; while it soon perished when 
attached to stems of the great tree cactus (Carnegiea gigantea) at 
less than 7 atmospheres (fig. 1). The last named plant exudes 
an acrid fluid from fresh wounds, which are quickly closed by the 
formation of heavy, corky layers. Opuntia versicolor, a species 
with cylindrical stems showing a pressure of 12 atmospheres, was 
able to draw supplies for extended periods from Carnegiea and 
the other hosts mentioned. Plantlets of A gave were equally effi- 
cient, although this xeno-parasite formed such a great number of 
Foots as to destroy the tissues of the host plant. Cissus also 
formed roots which penetrated the host, while the absorptive 
contact of Opuntia (flat-stemmed) was by the epidermal cells of the 
stems in every case examined. 
The briefest inspection of the results of the analysis of plants 
used in these experiments, shows that the direct proportion of 
mineral salts in the sap and the acid contents of the sap have no 
direct bearing on possible parasitism among the higher plants." 
The relation of seasonal cycles, capacity for development of absorp- 
tive elements de novo, and an accommodative adjustment of the 
Osmotic pressure of the cell sap are to be mentioned as factors in 
the making of nutritive couples. The greater number of the para- 
sitic arrangements made are to be included with the root parasites. 
A few additional experiments were set up to test certain points 
after the completion of the manuscript printed in 1910. The 
results of these and of the continuation of older preparations 
seem to warrant the presentation of this additional note on the 
subject. 
May 11, r910.—A number of beans of a mixture of species 
native to the deserts of southern Arizona were attached to joints 
‘See MacDovcat and Cannon, The conditions of parasitism in plants. Publ. 
No. 129, Carnegie Inst. of Wash. 1910. 
