DEPARTMENT OF BOTANICAL RESEARCH, • 59 



Mrs. G. J. Livingston's Annotated Bibliography of Evaporation has now 

 been completely published in the Monthly Weather Review, and separate re- 

 prints have been issued. 



Conditions of Parasitism (by Dr. D. T. MacDougal and Dr. W. A. Cannon) : 



The long series of experiments upon this subject show that it is possible to 

 establish regenerated cuttings of a number of species in a dependent nutri- 

 tive relation with the bodies of enforced host-plants. 



Arrangements of xeno-parasitism were made which endured for two 

 seasons or more. The xeno-parasite formed roots which penetrated the tis- 

 sues of the host in some instances, while in other cases absorption occurred 

 through the epidermal tissues of the submerged bases of the inserted slips. 

 The facts at hand do not warrant any conclusion as to the significance of 

 morphological features in the assumption of nutritive relations between two 

 seed-plants. 



The development displayed by xeno-parasites was in all instances less than 

 that of similar shoots autophytically nourished. The atrophy of the shoot 

 characteristics of parasites was thus displayed as an immediate response to 

 dependent nutrition. In addition to manifestations which might be classed 

 as direct responses, the etiolated shoots of Opuntia exhibited striking auto- 

 nomic movements not attributable to inequalities of growth. These move- 

 ments appear to be caused by a rhythmic inequality of turgidity in the outer 

 parenchymatous tissues of the stems, which undergo changes simulating 

 decortication. The zones of curvatures producing the movements moved up 

 and down the stems from base to apex in a period of about 72 hours. 



Successful xeno-parasitism is dependent in the first place upon the supe- 

 rior osmotic activity of the parasitic member of the nutritive couple, although 

 not all pairs of plants sustaining such inequality are capable of becoming 

 host and dependent, and other features act as limiting factors of minor im- 

 portance. The proportion of salts dissolved in the sap of the experimental 

 plants and the osmotic activity as indicated by freezing-point tests undergo 

 wide seasonal variations, as a result of which a xeno-parasite may maintain 

 itself upon a host during the period of greatest turgidity of the latter and 

 be unable to withdraw material from it during the drier season, when the sap 

 of the host is of a relatively greater concentration. The general conditions 

 which govern the origination of parasitism are thus so well apprehended 

 that, given any two plants, knowledge of their capability for entering into a 

 nutritive couple may be put in the form of an algebraic equation, the reduction 

 of which would indicate with some certainty the possibility of their adhesion. 



The unceasing distributional movements of plants would operate to bring 

 under test conditions a large number of pairs of species, and it seems quite 

 reasonable that new parasitic unions are being constantly formed in almost 

 all kinds of habitats. This fact might escape detection by ordinary methods 

 of observation almost indefinitely. The detailed description of the investiga- 

 tion of the entire problem is given in Publication No. 129, recently issued. 



