494 



LECTURE XXIX. 



ventral one, one side of which is differently organised from the other. Finally, 

 we may suppose yet a third construction : assuming some hundreds or thousands of 

 seedlings, each of which possesses a radicle and a plumule, to be placed beside one 

 another and parallel in the ordinary direction, and closely connected together, then 

 these seedlings form a whole in the direction of a line or in a surface, all the-roots 

 being on the one side, all the shoots on the other. We have then a dorsi-ventral 

 structure, one side of which is composed of roots and the other of shoots. And con- 

 versely, many dorsi-ventral organs may also be so broken up into individual elements 

 that each of the latter possesses a root-end and a shoot-end ; if, for example, a piece 

 is supposed to be cut out of the horizontal flat shoot of a Marchantia, perpendicular 

 to the surface, by means of a narrow cork-borer for instance, this portion bears on 

 the under side one or several rhizoids, and on the upper side cells containing chloro- 

 phyll, and it behaves like a small upright plant. Similarly we may isolate from a 

 . Marsilia or Pilularia, by means of two transverse sections of the shoot-axis, a so-called 

 node, on which a leaf is situated above, a root below. That such a mode of looking 



at dorsi-ventral structure is not an 

 arbitrary playing with words, but, 

 on the contrary, accords with the 

 true being of such a plant, fol- 

 lows, as I have definitely shown 

 previously, from the behaviour 

 which dorsi-ventral organs ex- 

 hibit towards the influence of 

 gravitation and light : from every 

 dorsi-ventral organ which places 

 itself across the direction of gra- 

 vity and of the rays of light, and 

 is thus plagiotropic, a radial or- 

 gan can be produced simply by 

 rolling it together, and this, under 

 the influence of gravitation and 

 light, behaves as a radial orthotropic organ. Nature herself makes this experiment 

 in cases where young foliage leaves are rolled round one another in the bud and 

 constitute a radial orthotropic structure ; when the older leaves become freed from 

 the bud and unroll, they then appear as dorsi-ventral organs which place themselves 

 across the directions of light and gravitation. 



From what has been hitherto said it obviously follows that the radial or the 

 bilateral dorsi-ventral organisation of a plant-orga,n constitutes a fundamentally primary 

 property of it, upon which depends not only the . reaction towards external directive 

 forces, but also the ways and means by which new formations — secondary ofi'shoots— 

 proceed from a given organ. The arrangement of the leaves and lateral shoots on a 

 shoot-axis is above all determined by whether the latter itself possesses radial bilateral 

 or dorsi-ventral structure ^ In this simple point lies at once the complete refutation 



Fig. 329. 



' The statements of fact here referred to are borrowed from Goebel's work, ' Ober die Ver- 

 zweigung dorsiventraler Sprosse ' (Arb. des hot. Inst., B. II, pp. 353, fee). I need only remark that 



