MOVEMENTS OF SHOOT- AND ROOT-FORMING SUBSTANCES. 521 



detail in my treatises on ' Stoffund Form der Pflanzentheile S' two causes, which usually 

 co-operate, come into consideration here : on the one hand it is to be noticed that 

 the substances for the formation of organs (apart from germinal stages where they 

 proceed from the reservoirs of reserve-material) are produced in the foliage leaves by 

 assimilation, and pass out thence into other parts of the plant. So long as a vigorous 

 main bud of the shoot is present, mixtures of substances suited chiefly for the 

 formation of shoots pass into it from the leaves, whereas the substances fitted for the 

 formation of joots flow in the opposite direction, into the roots which already exist. 

 If then a portion of the shoot-axis is cut off and kept in a moist warm environment, 

 the substances suitable for the formation of shoots which are already present in it 

 will, as heretofore, move in the acropetal direction, and those which form roots, on the 

 contrary, in the basipetal.one — i. e. the shoot-buds will make their appearance on a 

 cut-off piece of stem at the apical end, the young roots at the basal end. Exactly the 

 reverse must take place, however, in the regeneration occurring in a cut-off piece of an 

 old root : since here the root-forming substance is in continual movement towards 

 the apex, the roots produced by regeneration will arise at the acropetal end, and if 

 any bud-forming substance at all has reached the roots it may come forth at the 

 basal end of the piece of root, if the second external cause to be mentioned hereafter 

 co-operates. That roots as well as shoot-buds appear at the basal end of a cut-oflF leaf 

 is explained, according to this view of the matter, simply by supposing that during 

 the assimilatory activity of the leaf, and in accordance with its normal function, 

 shoot-forming as well as root-forming substances are continually passing from the 

 leaf through the petiole into the shoot-axis. If then the petiole is cut off, this customary 

 movement proceeds no further, being, as in the case of cut-off pieces in general, to a 

 certain extent checked at the section, and buds as well as roots will grow out from 

 the petiole near the section. These considerations then show that effective causes 

 are given in the organisation and vital activity of the plant itself by which the places 

 of origin of new roots and shoots are determined. 



A second cause, however, has already become known to us in the examples 

 quoted above; this is the influence of gravitation, which, so far as we are as yet 

 informed, affects different plants in very different degrees, in that new shoot-buds 

 arise more easily in cut-off portions on the ends directed upwards, new roots on 

 those directed downwards. If we suspend a cut-off piece of a shoot-axis in moist air, 

 with its basal end downwards, two influences are working together to induce roots to 

 arise at the latter, and at the same time to cause shoots to spring from the acropetal 

 end. If, however, we give the cut-off piece the reverse position, with the acropetal end 

 downwards, the two causes mentioned must work in the opposite way : the internal 

 disposition in this case induces the formation of buds at the acropetal, lower end, 

 while the influence of gravitation strives, so to speak, to draw the root-forming 

 substance downwards, and to push up the shoot-forming substance to a correspond- 

 ing extent, and thus bring about the opposed position of the new organs. It depends 

 now entirely and simply upon the reactive capacity of the plant concerned, whether 



' That In regeneration occurring in cut-off parts of plants an internal disposition depending on 

 the organisation of the plant co-operates with a direct effect of gravitation and other forces, I have 

 shown in detail in Arb. des bot. Inst. B. II, pp. 691, &c. 



