520 



LECTURE XXXI. 



If however cut-off segments of Opunlia are set with the apical end (a piece of which 

 has been cut off transversely) in the soil, in such a manner that the basal end is again 

 turned upwards but projects into the air, two to five months pass by before any 

 roots make their appearance from the apical end of the shoot segment, thus 

 directed downwards. We may explain the result of this experiment by the assumption 

 that root-forming substance exists in the shoot-segments of the Opuntia, but that in 

 the normal position of the plant it always seeks to move in the direction of the base, 

 in order to strengthen the subterranean root-system of the plant. By this is produced 

 a predisposition in the tissue of the shoot-segments which facilitates the movement 

 of the root-building substance towards the base, whence is explained that also in 

 inverted shoot-segments — but with the base planted in the soil— new roots are pro- 

 duced rapidly and vigorously at the basal end. If on the contrary the apical end of 

 the shoot-segment is planted in the soil, and at the same time turned downwards, it 

 requires a long time before the root-forming substance is able, in opposition to the 

 previous predisposition and in obedience to gravitation, to make its appearance at 

 the apical end in the form of roots. That this happens at all in the present 

 case is essentially aided by the additional influence of darkness and moisture, which 

 promote the formation of roots at the planted apical end. 



In the experiments last quoted we have to do in the main with the reproduction 

 of organs on separated portions of plants, concerning which numerous experiments 

 have for a long time been to hand, and which have been recently investigated by 

 Voechting ^, but in part incorrectly explained. Practical gardening has known from of 

 old the fact that it is possible to obtain new plants from even small portions of older 

 ones, roots and shoot-buds being formed on them. For this purpose it is usual to lay 

 the cut-off pieces in moist sand or soil ; for scientific purposes, however, it is often 

 better, as Voechting did, to suspend the separated portions in moist air, in order to 

 attain an equable environment favourable to growth. If these cut-off pieces of the 

 shoot-axes of woody or herbaceous plants are suspended in a horizontal or vertical 

 position, the usual result is that roots arise from their basal ends, and shoot-buds 

 from their acroscopic or apical ends — from which however the true apex has been 

 cut off. As a rule the behaviour is the reverse in the regeneration of cut-off pieces 

 of root; here the tendency prevails to produce shoot-buds at the basal end, and 

 roots at the acropetal end — i.e. the end turned towards the growing-point. 

 Finally, if the same experiments are made with cut-off leaves, buds and roots arise 

 at the base of the cut-off petiole, the buds usually being turned upwards, the roots 

 downwards. Voechting has attempted to bring the results of numerous experiments 

 in this direction which he has made under a general expression, intended to mark 

 everywhere a distinction between what he terms the apex and the base of the re- 

 generating organ. It is obvious, however, that in this sense the facts shortly referred 

 to above cannot be collated at all. According to my view, on the contrary, it is easy 

 to get at the heart of the matter if we proceed from certain allowable assumptions, and 

 keep in view the immediate effective causes which determine that roots and shoots must 

 arise at definite spots on cut-off parts of plants. In fact, as I have clearly shown in 



' Voechting, 'Über Organbildung im Pflanzenreich,'' Bonn, 1878. Compare my reply in the 

 treatise quoted in note i (p. 517). 



