Igit] MACDOUGAL—PARASITISM 253 
and one showed a second pair. The others still retained the plum- 
ule partly between the cotyledons. 
August 2.—All beans were dead or nearly so. All of the devel- 
opment in these plants was undoubtedly carried on at the expense 
of material in the cotyledons; and the roots soon perished after 
being immersed in the mucilaginous tissues of Opuntia or the 
stems of Oenothera. The high humidity of the fogs and low 
temperatures ranging between 45° and 65° F. also made for the 
endurance of the seedlings. These tests are chiefly interesting 
in contrast with the cultures of Prrrce, in which plants of Pisum 
salivum were grown on stems of Vicia Faba to maturity. The 
advantage of the aeration of the roots in the central cavities of 
the stems of the host and also of one legume parasitic upon 
another, doubtless accounts in large measure for the success of 
' these cultures. 
The completion of the original manuscript on this subject 
left several preparations in good condition, which were two years 
old. Among these were Opuntia Blakeana, O. versicolor, O. arbus- 
cula, and O. leptocaulis on Carnegiea; O. leptocaulis on O. discate; 
also one Agave americana on Carnegiea. Some of these parasites 
remained alive throughout a part or all the year, it being noted 
that those shielded from direct illumination by the body of the 
host survived longest. March ro11 found arrangements of Opuntia 
versicolor, O. Blakeana, and A gave on Carnegiea. 
All these preparations were made with plants as xeno-parasites 
Which were characterized by a water balance of some amount and 
by an osmotic pressure of the sap of 9-12 atmospheres. Further- 
more, the survivors were held in place by a mass of plaster of 
Paris molded about the bases of the stems which held the roots 
closely appressed against the corky tissues of the host. 
This state of affairs may be seen to furnish a fair approximation 
of the physical conditions under which an Opuntia was found with 
the roots in a small cavity in the trunk of a Parkinsonia, and of the 
Same species in a cavity in the summit of a trunk of Carnegiea.° 
A further illustration is offered by a case photographed and reported 
* Peirce, G. J., Artificial parasitism, etc. Bot. Gaz. 382214. 1904. 
3 See Publ. No. 129, Carnegie Inst. of Wash. t1gTo. 
