504 LECTURE XXX. 



From this point of view, even the rudimentary beginnings of a causal explana- 

 tion of the processes of organic growth appear of importance, even though we are not 

 in a position to explain in any given case the connection of cause and effect link by 

 link and step by step. It suffices that it can be said at all definitely that this or that 

 relation of configuration in the province of organic beings may be referred to definite 

 causes. 



As already mentioned, then, we are in this lecture concerned with those causal 

 factors, which by the growth of one organ of a plant are given for the growth of 

 another organ of the same plant ; or, to put it shortly, with the correlation of growth 

 of the different organs of a plant. 



A very plastic subject for experimental researches in this province is the 

 common Potato. On the subterranean portion of the shoot-axis, which developes 

 above ground as the leaf-shoot, there arise in the axils of small scale-leaves in the 

 normal course of things, thin filiform horizontally spreading subterranean shoots, 

 which likewise only produce small scale-leaves, and which finally give rise to the 

 potato-tuber, by vigorous growth in thickness at the tip of the shoot-axis. If at the 

 period when the formation of tubers has not yet commenced, the portion of the leaf- 

 shoot which is above ground is cut off, the terminal buds of the still young filiform 

 runners become converted into ordinary leafy shoots, which ascend and grow out 

 above the surface of the soil. Thus by removing the young main shoot it is possible 

 to cause its lateral shoots, which would otherwise form tubers, to assume an entirely 

 diflferent form of growth ; and we may therefore infer that in the ordinary course of 

 the vegetation of this plant, the growth of the foliage-shoot brings it about that its 

 subterranean lateral shoots become, not foliage-shoots likewise but potato-tubers. It 

 is also possible to cause the production of tubers at will on the sub-aerial leaf-shoot, 

 if the subterranean lateral shoots destined for the formation of tubers are carefully 

 cut away from a vigorously growing potato plant, and the possibility of the formation 

 of tubers below ground prevented. The materials normally adapted for the formation 

 of potato-tubers now pass into the axillary buds of the sub-aerial foliage leaves, and 

 cause their axial portions to remain short and to swell up and thicken, while their 

 leaves develope but feebly. The existence of the subterranean runners, then, brings 

 it about in the normal course of affairs that the materials destined for the formation 

 of tubers do not prevent the development of the sub-aerial buds into leafy shoots. 



No less evident and easy to observe is a similar correlation between the apical 

 shoot of many trees and the lateral shoots beneath the apex. If the terminal shoot 

 of the Fir A6ies excelsa and several species of Abies allied to it (e.g. A. cephalonica) 

 is broken off, or destroyed by frost, &c., the horizontal lateral shoots of the upper- 

 most whorl gradually erect themselves, and occasionally a similar effect is ob- 

 served even on lateral shoots of the next lower whorl. After 1-3 years one of these 

 lateral shoots has usually obtained the upper hand, and has not only become erected 

 vertically, but has also lost its bilateral nature ; the originally horizontal shoot has 

 gradually become radial and completely orthotropic, and it produces henceforth four- 

 or five-rayed whorls of branches exactly as the original terminal shoot of the main stem. 

 We may conclude from these facts that in the normal course of growth a causal 

 relation exists between the growth of the young lateral shoots and that of the apical 

 shoot of the main stem ; the growth of the latter evidently brings it about that the 



