The Making of Parasites. 21.^ 



reduction of which should indicate with some certainty the possi- 

 biHty of dependent nutrition between two plants. 



The unceasing distributional movements of species over 

 the face of the world would undoubtedly operate to bring new 

 pairs of species constantly under test conditions, and it is in 

 accord with the known facts to suppose that new parasitic unions 

 are being formed frequently in almost all kinds of habitats, while, 

 on the other hand it is to be kept in mind that extinction Hes 

 beyond parasitism, as the inevitable atrophies which follow 

 dependent nutrition must in the end bring the species to a pre- 

 carious condition in which the slightest untoward variation in its 

 highly complicated environment would end its existence. 



The changes ensuing when a plant becomes parasitic mav 

 include some startling phenomena, as was illustrated bv the ac- 

 tion of the prickly pear used in the experiments. The structure 

 of the heavy .succulent, jointed stems of the prickly pears pre- 

 sent no features which suggest movement, yet when etiolated 

 cylindrical branches of these plants a few inches long were in- 

 serted in the fleshy bodies of the barrel cacti, and had become 

 well established, sweeping nutatory movements like those of a 

 vine were set up which carried the tips through an arc of 180 

 degrees in 36 hours, the complete nutation occupying twice that 

 length of tim.e. Furthermore, these movements differed from 

 other nutatory movements hitherto described by the fact that 

 they were not accompanied by growth in length. So far as could 

 be determined, the curvatures resulted from rhythmical altera- 

 tions in the turgidity or osmotic activity of the cell-sap on 

 opposite sides of the thin stems, thus setting up a movement ut- 

 terly foreign to the ordinary habits of the plant, and purposeless 

 so far as its environmental relations were concerned. 



The results of this and previous studies allow us to recog- 

 nize the limiting factors, the operative conditions, and some of 

 the facts as to the physiological nature of parasitism. Few 

 specializations present their main features so clearly to the 

 student. When the total body of evidence, however, is evaluated 

 in the light of current theories, it is found that it is no easy matter 

 to decide by what main methods of evolutionary procedure this 

 adjustment or "adaptation" is attained, and how it is ad- 

 vanced from stage to stage. 



