222 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



of a twig of poplar. There is formed here first of all, under favourable 

 external conditions and in consequence of the wound-stimulus, a growth 

 of tissue on both cut surfaces a callus. What happens subsequently 

 may be shortly told in accordance with the observations of Tittmann l 

 upon Populus pyramidalis and P. nigra. The tendency to formation of 

 roots in the severed portion of shoots of these plants is not great. The 

 formation of callus is quite independent of gravity. Erect cuttings form 

 at the apical end a massive callus which bears many adventitious shoots, 

 the basal end either remains without any new formations at all or 

 produces roots. This corresponds with what is usual in regeneration, 

 as has been explained above, and is the result of the direction of the 

 current of plastic material in the uninjured plant. Shoots arise at the 

 apical end in conformity with this ' internal disposition.' But external 

 influences are operative, for shoots arise at the upper end, roots at the 

 loiver end. This happens even if the portion of shoot should be 

 inverted, and have its real apical end in the sand and its basal end 

 upwards. In such circumstances the formation of callus was found to be 

 dominant on the basal (free) part, and shoots appeared upon this in 

 about half of the cuttings ; the downwardly directed apical part 

 remained without new formations or produced shoots also. Here then 

 in consequence of inverted position shoots appeared upon the basal end 

 of the shoot, but the ' internal disposition ' of the apical end to form 

 shoots could not be entirely suppressed. Suppression may however come 

 about through correlation. If, for example, the apical end stands in dry air, 

 whilst the basal end is in water, the latter being under much more 

 favourable external conditions forms the shoots and thereby hinders 

 the formation of shoots at the apical end. 



The influence of gravity upon the formation of new shoots is quite 

 evident in the example just mentioned formation of shoots on the up- 

 turned end is favoured. It also becomes noticeable in the formation 

 of organs upon cuttings provided with primordia of shoots, or with prim- 

 ordia of shoots and primordia of roots, although here in less striking 

 degree. Du Hamel long ago observed that on twigs of willow laid 

 upon the ground the roots only appeared upon the under side. 



The influence of gravity is felt also in the formation of roots in 

 uninjured herbaceous plants. Suppose we take shoots of Tropaeolum 

 majus and, without severing them from. the mother-plant, lay them in vertical 

 and in horizontal positions and cover them with earth so that the long 

 stalked leaves remain in the light, it will be found that the roots sprout 

 out on all sides of the vertical shoots, whilst they appear only on the 



Tittmann, Physiologische Untersuchungen iiber Callusbildungen an Stecklingen, in Fringsh. 

 Jahrb. xxvii. p. 164. 



