78 REGENERATION 



of oxygen in the water, and as a result the buds are prevented from 

 developing. 



It can be shown that light has also an influence on the regen- 

 eration of pieces, and that it has a stronger influence on some plants 

 than on others. In some plants roots develop only on that side of 

 the stem that is less illuminated. In Lepismium radicans, for in- 

 stance, adventitious roots are produced by the plant even in dry air. 

 Pieces of the stem can produce roots on either the upper or the 

 lower surface, according to which side is less illuminated. A 

 piece of the stem of this plant that had been kept in the dark pro- 

 duced two roots, one above and one below, one, therefore, opposed 

 to the direction of the action of gravity, and the other in the direction 

 of that action. Even in pieces of the willow, suspended in a moist 

 atmosphere, roots develop better and over a greater length of the 

 stem on the less illuminated side. 



Although the experiments with pieces of young willow-twigs may 

 seem to show that gravity is not a factor in regulating the develop- 

 ment of the new parts, the results show in reality only that internal 

 factors have a preponderating influence. By means of another series 

 of experiments it can be shown that gravity does have an influence on 

 the production of the new parts. It is evident that in order to test 

 the action of gravity, pieces must be placed in different positions in 

 relation to the vertical. It will be found, if this is done, that different 

 results are obtained according to the angle that the piece makes with 

 the vertical. If a piece is suspended in a moist atmosphere, with its 

 apical end upward, the smaller the angle that the piece makes with 

 the vertical so much the more are the leaf-buds that develop confined 

 to the upper part of the piece, and so much the more do they develop 

 from all sides of the upper end ; conversely, the greater the angle 

 with the vertical, i.e. the more nearly horizontal the position of the 

 piece, so much the more are the leaf-buds that develop found along 

 the upper' side of the apical end (as well as around the end). If the 

 piece is placed in a horizontal position, the leaf-buds develop not only 

 around the apex, but they develop along the entire length of the upper 

 surface, best, however, near the apical end. 



If similar pieces are suspended in oblique positions, with the basal 

 end upward, different results are obtained. In the preceding experi- 

 ment the polarity of the piece and gravity act together, while in this 

 experiment their action is opposed. Although there is a great 

 amount of variability in the results, yet the action of gravity is found 

 to have less influence on the result than has the inner polarity, and 

 the influence of the latter is so much greater that the action of gravity 

 is hardly noticeable. 



The roots do not show as markedly the influence of gravity as 



