62 REPORTS ON INVESTIGATIONS AND PROJECTS. 



water, may be determined. This apparatus is a modification of the method 

 commonly in use for the determination of the same factors, but is a shorter 

 as well as a more convenient one. The special form of receptacle employed 

 in the cultures permits the observation of the rate of the growth of the roots, 

 and the use of air either dry or moist, as well as the application of heat or 

 cold as desired. In addition to these pieces of apparatus a clinostat for 

 carrying duplicate cultures weighing 70 pounds or more was also designed. 

 A few preliminary experiments indicate that the roots of such plants as 

 the mesquite (Prosopis) and the ocotillo (Fouqueria), the former with a 

 variable root-system, which may or may not penetrate deeply, and the latter 

 with a root-system relatively shallowly placed and with little variation, have 

 unequal responses to the air-relation. Under similar conditions of aeration 

 the roots of the mesquite grow more rapidly than those of the ocotillo. The 

 rate of growth of the latter in particular, as well as the richness of branch- 

 ing, is directly associated with aeration. When an excessive amount of air 

 is presented to the roots, however, retardation of growth takes place. The 

 results of similar experiments on mesophytes indicate that increased aeration, 

 within limits, favors root-growth and shoot development. Roots appear to 

 grow equally well in sterilized and in unsterilized soil, under similar condi- 

 tions of aeration. In unsterilized soil, however, soil organisms develop more 

 rapidly with artificial aeration than without, but the position occupied by 

 them in the soil appears to coincide with the extension of the root-system. 



Structural Relations in Xenoparasitism, by W. A. Cannon. 



Artificial parasitism has been induced in several species by Dr. D. T. 

 MacDougal, who has reported on them from time to time (Carnegie Insti- 

 tution of Washington Publication 129). Among the nutritive couples, and 

 one of the most successful, was that of the Mexican grape (Cissus digitata), 

 which was induced to form roots and a shoot with leaves and tendrils when 

 attached to a flat-stemmed opuntia. The structural relations of the parasite 

 and host in this nutritive couple have been investigated by Dr. Cannon. 



The xenoparasite-organized roots within the tissues of the host totaled 

 over 7.5 cm. in length, all of which at first was capable of absorbing nutri- 

 ment from the host. The older portions of the root, however, were found 

 to have organized a layer of cork. The portions of the host adjacent to the 

 fresh-growing parasitic roots were of unmodified parenchymatous tissue, 

 but wound-tissue is soon formed, so that the living cells of the host and those 

 of the parasite are ultimately separated from each other by non-living tissue 

 of considerable thickness. When the latter is the condition, diffusion from 

 host to parasite would take place very slowly, if at all. 



The character of the responses of the root to the host is suggestive of 

 possible changes occurring in the initial steps of parasitism of habitual para- 

 sites. In the xenoparasite root-hairs were not formed, indicating that 

 atrophies of different root-organs are among the first structural changes to 



