DEPARTMENT OF BOTANICAL RESEARCH. 81 



and method of pollination, the rate of extension, and course of the 

 pollen-tube, as well as the general structure of the ovule. (Fig. 4.) 

 Duplication of results within narrow limits is not to be expected. An 

 uncertain number of ovules are affected in any operation, the egg or 

 sperm nucleus may be affected singly, or the fertilized egg may be 

 reached by various concentrations of the reagent. 



(7) The results of these experiments justify a consideration of the 

 germ-plasm of plants as a concrete complex having two distinct phases, 

 not as an "idealoplasm." In one phase it is the meristem or embry- 

 onic tract splitting off cells which pass into permanent tissues; the form 

 and character of these differentiated cells may be greatly modified by 

 environic forces, which, however, make no permanent impress on the 

 germinal tract. The embryonic tract at times gives rise to specialized 

 sexual reproductive elements, representing the second phase, during 

 which process a halving of the number of chromosomes takes place. 

 Subsequent to these reduction phenomena, the germ-cells may be 

 profoundly influenced by extrinsic substances in the manner described 

 above. 



Relation of Root-habit to Soil Temperature, hy W. A. Cannon. 



Studies on the root-habits of perennials, which have been carried on 

 at the Desert Laboratory, have revealed striking differences in root- 

 development among species living in close proximity and apparently 

 subject to similar environmental conditions. For example, Rrosopis 

 velutina has a root-system which may penetrate the soil deeply, while 

 the roots of Fouquieria splendens and Opuntia versicolor and other 

 cacti rarely, if ever, attain a depth exceeding 40 cm. On the other 

 hand, it has been learned that the two species last named, under certain 

 cultural conditions, may send their roots to a depth of a meter and 

 more. A tentative explanation, based mainly on experimental studies 

 of the leading causes which operate to bring about the results noted, 

 will be offered in the succeeding paragraphs. 



As already stated in another section of this report, the temperature 

 response of desert perennials is unlike. For example, the temperature 

 suitable for effective growth-rate of the roots of Opuntia and Fouquieria 

 is relatively high and lies above 21.5° C, while the roots of Prosopis 

 grow fairly actively at a temperature 5° C, or more, below this. It 

 is thought that this unlike response to the temperature of the soil, 

 taken together with the seasonal march of soil temperature at various 

 depths, may be the chief causes which determine the type of root- 

 development characteristic of the species. 



An important feature of the soil temperature at a depth of 15 to 

 30 cm. is the great difference to be found in the amount of heat between 

 winter and summer. This appears in an especially striking manner 



