64 REPORTS OF inve;stigations and projects. 



ogy, the relations of the character of the root-system to the distribution of the 

 plants and to their general habits, and so far as possible experiments designed 

 to show the factors by the action of which the leading characters of the roots 

 have risen. 



Among the recent noteworthy observations on the habits of the roots of the 

 cacti may be given the discovery of vegetative reproduction in Opuntia arbus- 

 cula (in the region about Tucson) through its fleshy roots, and the fact that 

 the form of this species growing in the vicinity of Sacaton, Arizona, is not 

 provided with fleshy roots. 



The Physiology of Genetics. — The experiments of Dr. D. T. MacDougal, 

 by which the reproductive elements of the higher plants were subjected to the 

 action of chemical compounds of various kinds, inducing breaks in heredity, 

 have been continued and some progress has been made both in the effective- 

 ness of the methods used and in the analysis of the results. 



The general method of operation consists in injecting solutions of calcium, 

 potassium, iodine, or zinc into ovaries at some time within 24 hours of the 

 advent of the pollen-tube and the accomplishment of fertilization. The re- 

 agents in plants such as Cereiis were found to be absorbed by the walls of the 

 cavity from which the stalked egg-apparatus arises and through which the 

 pollen-tube must pass in reaching the eggs. By the use of staining solutions 

 it was seen that the reagent in discernible quantity penetrated as far as the 

 outer part of the inner integument of the embryo-sac nearest the antipodal 

 cells, from which it was separated by several protoplasts. More effective con- 

 centrations are brought in actual contact with the thin walls of the advancing 

 pollen-tube so that the weight of probability at the present stage of informa- 

 tion suggests that the method actually influences the pollen-cell rather than the 

 embryo-sac, although the reverse may be true in some species not yet exam- 

 ined as to this point. Breaks in heredity, discontinuous inheritance, or muta- 

 tions have been taken on theoretical grounds to consist in changes ensuing 

 previous to the reduction divisions in the reproductive elements, but the effects 

 secured by this method result from action after the second or third division 

 following reduction division and after a comparatively long period of time. 

 The results in question may be more similar to those shown in sports or bud- 

 mutations than those of seed-mutations. 



The striking form originally secured from Oenothera biennis by this method 

 has been cultivated through several generations, transmits all of its characters 

 fully, and does not readily hybridize with the parent. 



Great difficulty was experienced in securing proper germination of the forms 

 treated in the early season of 1907, but of the seven species experimented with 

 Cereiis giganteus yielded a progeny some of the individuals of which depart 

 far from the usual type. The most striking results, however, arc obtained 



