DORSIVENTRAL LATERAL SHOOTS 93 



The chief shoots of our species of oak 1 Ouercus pedunculata 

 and O. sessiliflora are radial with a 2 / 5 phyllotaxy, but their lateral 

 shoots are dorsiventral, for the insertion of the leaves upon them is not 

 transverse but inclines to the long axis of the shoot, in the same way 

 only not so strongly as in the distichous shoots of Tilia, Fagus, and others 

 (Fig. 49). If a shoot be looked at from above, it will be seen that in the 

 leaves upon the left side of the shoot the left margin is deeper than 

 the other, whilst in those upon the right side the right margin is deeper. 

 Leaves which are inserted quite on the dorsal side often show a transverse 

 insertion and the degree of the obliquity of the insertion of the lateral 

 leaves varies greatly. The leaves on the dorsal side of the shoot have 

 also nearly equal sides, whilst on the others the sides of the leaves are 

 differently formed. Anisophylly, about which more will be said presently, 

 is then noticeable in the lateral shoots of the oak and these have passed 

 over in the course of their development from a completely radial prim- 

 ordium to a dorsiventral construction which however is not strongly 

 marked. 



In lateral shoots the leaf-insertions are often brought by torsion 



of the internode into one plane, which is nearly 



horizontal. The upper surfaces of the leaves are yo vo .0 

 thus turned towards the light 2 . This is very ^- ^ 



strikingly the case in shrubs whose leaves are FIG. 49. Scheme of the insertion 



of leaves and lateral shoots on the 



arranged in decussate whorls, such as species of dorsiventral branches of Tiiia, 



Fagus, and others. 



Lonicera, Philadelphus, Deutzia, but it is also 



found in some which have spiral phyllotaxy, for example, species of 

 Spiraea. I have elsewhere shown 3 that orthotropous shoots which become 

 plagiotropous in feeble illumination from one side exhibit like features, 

 that is to say, a torsion of the internode brings the leaf-surface into 

 a position at right angles to the incident light ; Gentiana asclepiadea 

 shows this very clearly when growing at the edge of woods, and it occurs 

 in other plants also. The torsion which brings the leaf-surfaces nearly 

 into one plane is in other cases effected in the leaf-base itself, in the 

 needles of the silver-fir, for instance, but this does not affect the final result. 

 In the Coniferae this phenomenon is connected with the non-development 

 of internodes, and in them the dorsiventral character of the lateral shoots 

 is particularly evident when they branch. The branches of higher order 

 arise exclusively or at least preferably from the flanks of the lateral 

 shoots, and in this way the flat branch-system which every one recognizes 

 in the silver-firs comes about. The influence of external factors upon 



1 See Wigand, Der Baum, p. 45 ; Mohl, Morphol. Untersuchungen iiber die Eiche. 



2 See Frank, Die natiirl. wagerechte Richtung von Pflanzenteilen. Leipzig, 1870. 

 '* Goebel, Zur Morphologic und Physiologic des Blattes, in Botan. Zeitung, 1880. 



