198 Guayule. 



basal shoots of mariola do normally, namely, each will send out a root 

 from near its base (plate 44, fig. A), which becomes, in effect, a tap-root. 

 Thus these branches become established independently of the parent stock, 

 and may be separated from it and used for propagation. This occurs in 

 nature very exceptionally , but more readily when the top has been removed 

 by design or accident; in the field, however, roots are normally produced 

 from the basal portion of the retono chief shoot (fig. n), from which its 

 root-system proper is derived. The more ready production of roots in this 

 manner in irrigated plants is connected with the larger supply of water. 



It is seen that the guayule displays a marked polarity analogous to 

 that found in plants which will not regenerate roots from stems when ma- 

 ture, but will do so when young (Cupressus, vide Goebel, Organography, 

 Engl. ed., p. 45, I), and to certain lilies (Hyacinthus sp., Goebel, ibid.), in 

 which bulbils are formed from the lower portion of a severed stem, but 

 not above. That is to say, the expression of polarity by root-regeneration 

 from the stem is definitely restricted to a particular region of certain 

 stems only, namely, to the lowermost zone of the branches of the second 

 and (probably) higher orders, which themselves arise from a narrow zone 

 of the chief stem just above the tap-root. 



Shoot-regeneration is, by contrast, easy, and this is true for the root, 

 from which stem-primordia are absent. It does not appear that external 

 stimulus is necessary, for wounding the cortex of the root in situ is not 

 followed, in any of my experiments, by shoot-formation at the point of 

 wounding. Nevertheless, as in many plants, a complete severance of a 

 root left in situ is frequently followed by shoot-formation, but in a posi- 

 tion determined by other conditions, such as dying back, resulting from 

 drought. Thus it appears that the notion formulated by Miss Kupfer 

 (1907), that the disposition to form roots is much more generalized than 

 to form shoots, does not include cases like this before us, which need eluci- 

 dation as much as any. And as McCallum (1905) has well said, the prob- 

 lem of regeneration is more especially to determine the cause of non- 

 development "of parts" in the normal life of the plant. 



