THE ORIGINATION OF PARASITISM. 53 



included in the mycorrhizal forms, in which symbiotic arrangements 

 between fungi and roots result in forms of nutrition accompanied by alter- 

 ations in the shoot of the higher plant separable in neither degree nor 

 kind from those characteristic of parasitism. Marked developments of 

 mycorrhizal arrangements are known in a number of species comparable 

 to parasites, but the instances in which they are present in some incipient 

 form are so many that it would be justifiable to say that the adaptive move- 

 ment or adjustment by which higher plants use organic material derived 

 from other organisms is one which is manifested by perhaps half their total 

 number, making it evident that the feature under discussion is one of the 

 most important in the evolutionary development of the vegetable kingdom. 



The work being done upon the investigation of the water-balance of 

 desert plants suggested that these forms, particularly the succulents, might 

 well furnish objects advantageous for experimentation upon the subject. 

 The reservoirs of material in solution held by such plants would be expected 

 to furnish ready places of attack for possible parasites and, on the other 

 hand, various studies, the most recent by Kusano, show that parasites, 

 particularly the Rhinanthacese and Santalaceae, carry a large water-balance 

 in specialized tracheids. (Kusano, S., on the Parasitism of Siphonostegia, 

 Bull. Coll. of Agric, Tokyo Imperial University, vol. viii. No. 1, 1908.) 

 A water-balance is, therefore, in some cases at least, an accompaniment 

 and a condition of parasitism, and advantage was taken of the presumption 

 that its presence would be an experimental advantage. 



In the consideration of conditions which might operate to bring two 

 plants tog'ether in a nutritive relation, it was seen first of all that a cer- 

 tain general coincidence of habit was a prerequisite, in illustration of which 

 it may be pointed out that a slow-growing perennial could not flourish or 

 survive if attached to the body of an annual, or that a winter annual would 

 hardly fasten to the quiescent body of a perennial showing activity in 

 growth and food formation only in the summer. Furthermore, the oppor- 

 tunity for the formation of absorptive structures in contact with a possible 

 host would be of importance . Roots submerged in moist soil would naturally 

 afford the best conditions for such action, and the greatest number of 

 cases of parasitism among the higher plants are of the type in which under- 

 ground organs are united. The adhesion of shoots would not occur so 

 profusely, except as consequent upon the mechanical dependency of the 

 climbing or the twining plants, and hence it represents an advanced spe- 

 cialization when compared with the simpler forms of root adhesion. Om- 

 cuta offers itself as a well-known illustration of this type of developing 

 parasitism. 



The epidermal structures— coatings and bark — and the capacity of the 

 cortex for the formation of wound-tissues or dense secretions are factors of 

 much moment in nutritive couplings, as has been amply demonstrated by 

 features of my own work which can not be described here. It is evident 



