EXPERIMENTAL CULTURES. 



Although the primary object of the present research was to learn the 

 most striking facts regarding the root-system of mature desert plants, as 

 a necessary preparation to later experimental work, problems arose dur- 

 ing the course of the study on which experiments were carried out intended 

 to be suggestive rather than necessarily conclusive. The experiments 

 naturally centered around the relation of the roots to water supply: (i) 

 as regards formation of adventitious and temporary rootlets; (2) the direc- 

 tion of growth or position of the roots; (3) as regards the quality of fleshi- 

 ness in the young as well as mature roots of certain opuntias. 



The root-systems of many species of perennials and a few annuals are 

 provided with filamentous roots in groups of about 6 each. In perennials 

 these roots appear during favoring seasons and disappear when such sea- 

 sons have passed. In annuals their behavior is not so clear but their pres- 

 ence is probably also associated with an increase of the water supply. The 

 leading experiments undertaken along this line may be briefly stated. 

 Among the perennials, Franseria deltoidea and Encelia farinosa form tempo- 

 rary rootlets in midwinter, if the ground is moistened, but other species 

 such as Covillea tridentata, Fouqmena splendens, Lycium andersonii, and 

 Opuntia discata, are not provided at this season with newly developed 

 rootlets. Therefore, in certain of the species named, besides an improve- 

 ment of the water relations, a condition has to be fulfilled before the tempo- 

 rary absorbing roots may be formed, and this very clearly is higher tem- 

 perature, since freshly formed rootlets are to be found on these plants in 

 early autumn. However, such a plant as Opuntia discata can absorb water 

 in the winter season without the development of recognizably new rootlets, 

 as a series of experiments in the winter season well shows. 



On November 11, 1908, after several weeks of drought, a small specimen 

 of Opuntia discata was well watered, and the watering was repeated the 

 following day. Six measurements on the thickness of the flat joints were 

 made on as many different places; 31 hours after the water was applied 

 the joints had begun to increase in thickness; the increase in thickness 

 continued for three days, after which the maximum diameter was main- 

 tained for an unknown period. On the sixth day after irrigating, roots 

 of the plant were carefully removed from the soil and examined. No newly 

 formed rootlets were present. In summer, however, such roots are formed 

 on this species, and the vegetative activityof the plant is very great, as shown 

 by new growth, the formation of leaves, and a high rate of transpiration. 

 Among the annuals the behavior of the roots as regards the formation 

 of adventitious rootlets also appears to be varied, although probably con- 

 stant for any species. A large proportion of the annuals studied were seen 



