CHAPTER VIII. 

 VEGETATIVE REPRODUCTION. 



In attempting to solve the problem of the cultivation of a hitherto 

 totally feral desert plant, it became necessary to determine quantitatively 

 the possibilities of the plant for reproduction vegetatively as well as by 

 seed. As has been mentioned, the percentage of germination is small, 

 even under the best cultural conditions, so that any haphazard field 

 method of sowing seed, in the hope that nature will do the rest, is prac- 

 tically out of the question. In the hope that cuttings could be made to 

 grow readily and in sufficiently large quantities for cultural purposes, this 

 was gone into thoroughly. The net result of all the experiments is to 

 show that only a short zone of the stem is capable of root-regeneration, 

 namely , that immediately above the tap-root, but including some portion, 

 difficult to delimit, of the epicotyledonary region in seedlings and an anal- 

 ogous portion of the stem in retonos (fig. n). The ability to produce 

 roots in plants from seed is, however, not restricted to the main stem, but, 

 as will be shown, resides also in branches springing from the root-producing 

 zone. This fact is of rather special biological as well as economic interest, 

 and as it throws light on the failure of attempts to grow cuttings I shall 

 first present my observations leading to the conclusion stated. 



INDUCED ROOT-REGENERATION. 1 



Both the Mexican guayule (Parthenium argentatum A. Gray) and its 

 congener, the mariola (P. incanum H. B. K.), exhibit methods of vegeta- 

 tive reproduction which, while shared by other plants, are not common to 

 these under the normal conditions of growth. A somewhat detailed ac- 

 count of the matter has already been published, 2 but a brief restatement 

 will be necessary to make clear the point of the present discussion. 



The mariola is a low shrub with rather numerous branches rising 

 immediately from the base of the chief stem. These branches arise sub- 

 sequently to the development of the chief shoot, and not unusually, during 

 the first season of growth, from the seedling. Each following period of 

 development sees new lateral shoots of this kind arise again from the base, 

 either of the main shoot or, secondarily, from an already well-developed 

 basal-lateral shoot. Long continuation of this process results in the dense 

 group of stems arising near the surface of the ground which characterizes 

 the mature plant of the mariola. 



It is to be further noted that nearly all of these basal-lateral shoots 

 are provided with their own root-systems (plate 44, fig. B). From the 

 base of each new shoot, soon after it has accomplished a fair amount of 

 development, there spring adventitious roots, one of which, by the direc- 



1 Presented before Section G of the American Association for the Advance- 

 ment of Science at the Baltimore meeting, 1908. 



2 Lloyd, 19086. 



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