DORSI-VENTRAL SHOOTS. 



493 



upwards, and then not only all leaves, but also the lateral shoots springing 

 from the orthotropic main shoot, are dorsi-ventral in structure. This is unusually 

 clear in some Conifers, especially in the genus Aites: all the lateral branches 

 springing from the radially constructed stem, which grows perpendicularly upwards, 

 are here bilateral and dorsi-ventral in structure, and grow out in a horizontal 

 direction. Even in many Dicotyledons the same occurs, e.g. in the edible 

 Chestnut, the Red Beech and others, where the vertical main shoot exhibits 

 radial phyllotaxis and branching, but the 

 lateral shoots bilateral and dorsi-ventral 

 structure — bilateral branching combined 

 with a tendency to horizontal or oblique 

 direction. 



As follows from what has been stated 

 hitherto, the capacity of the organs of a 

 plant to react towards gravitation, light and 

 other directive forces, so that they grow 

 in a definite position of equilibrium with 

 respect to the horizon, co-operates causally 

 with their radial or dorsi-ventral structure ; 

 or, as we may also say, the Anisotropy^ 

 of the organs is determined at once by 

 their radial or dorsi-ventral structure. A 

 more detailed consideration of this matter 

 is better relegated, however, to the chapter 

 on the phenomena of irritability, though 

 on account of the close interdependence 

 between growth and anisotropy the subject 

 could not be entirely passed over here. 



For the elucidation of the most es- 

 sential difference between radial and dorsi- 

 ventral structure of an organ, however, 

 the following considerations may serve in 

 addition. Let us imagine an ordinary 

 foliage-leaf, in which the upper and lower 

 sides are sharply characterised, and which is therefore a distinctly dorsi-ventral organ, 

 so rolled together parallel to the longitudinal axis that its lower side appears as the 

 convex outer side. As Fig. 329 (C and A) shows, a radially constructed organ is thus 

 produced from the dorsi-ventral one ; since it is clear that a transverse section of the 

 rolled-up leaf must exhibit equal distributions of the organisation in all directions. 

 If, conversely, we suppose any hollow stem whatever slit up in the direction of its 

 length, and then spread out flat, there arises from the previously radial organ a dorsi- 



FIG. ^.—Pil-ularia globuti/era {nat. size) ; 

 s terminal bud of the stem ; ^ d' leaves ; -w roots ; 

 ysporocarps. 



* I first pointed out generally that Anisotropy, 1. e. the different capacity of the organs of the 

 plant to react towards similar external directive forces (gravitation and light), is a necessary result of 

 the radial or dorsi-ventral structure, in various editions of my Text-book, and then made it quite clear 

 in my treatise ' Ober orthotrofe vnd plagiotrofe Organe' (Art. des bot. Inst., B. II, pp. 274, &c ). 



