84 CARNEGIE INSTITUTION OF WASHINGTON. 



all pronounced. For two or more years shoot-growth in the species 

 was slight. In about three years from the seed the growth of the 

 shoot assumed a greater prominence, and it was found that lateral 

 roots had begun to develop in a pronounced fashion. With the pass- 

 ing of time the development of the laterals was more accentuated, 

 until at the end of about six years they constituted the most impor- 

 tant part of the root system. It is of interest to note the behavior of 

 the roots of Prosopis velutina, which was growing under the same 

 circumstances. In this a taproot was formed at first, but owing to 

 the low temperature of the soil at a depth of 1.5 m. it was of limited 

 growth. No laterals were formed the first or succeeding year. The 

 growth of the shoot stopped wdth the cessation of growth of the root 

 and the species failed to become established. 



Cultural experiments with species whose roots remain close to the 

 surface of the ground, and wdth characters apparently especially well 

 fixed, reveal a surprising plasticity in root development. This has been 

 observed in certain cacti, particularly in Opuntia versicolor. In garden 

 experiments with Opuntia carried out at the Desert Laboratory, where 

 fine and coarse soils were used, the usual type of root development 

 was seen in the finer soil, but there was a marked tendency to general- 

 ization in the roots growing in soil that was coarse. In other experi- 

 ments the roots of Opuntia were observed to assume the extreme 

 taproot habit, which under natural conditions never occurs. In these 

 experiments jointed terra-cotta and galvanized-iron pipes, 2.5 m. in 

 height and 7.5 cm. in inside diameter, were used. These were filled 

 with coarse soil and placed in an upright position with the lower end 

 resting in a large can. The pipes were thus exposed to the temperature 

 of the air throughout their entire length. Two specixaens of 0. versi- 

 color were planted in two of the pipes and one of 0. discata in the third. 

 When, after having been left growing for over three years, the cul- 

 tures were taken down, the following condition was found : The roots 

 of all of the 0. versicolor plants had attained to the bottom of the pipes 

 and had spread somewhat on the bottom of the supporting can. The 

 upright length of the roots was thus 2.5 m. There were some unim- 

 portant laterals. 0. discata formed several relatively slender roots of 

 about equal length, which were approximately 1.5 m. long. The soil 

 in all pipes was moist when the cultures were examined. Thus the 

 roots were well aerated and there was a high temperature of the soil 

 for the entire root-length during a portion of each year. It seems 

 highly probable, therefore, that if the soil in the habitats where these 

 cacti grow were at a depth of 3 m. sufficiently warm, adequately aerated, 

 and at the same time moist, deeply penetrating roots would be formed 

 by them. And, finally, it can not be said that in the cacti the dis- 

 tinctive root characters result from inherited tendencies, as is the 

 case in Prosopis velutina, for example, but, on the contrary, that they 



