INTRODUCTION 7 1 



tichously-leaved lateral shoots becomes vertical through torsion of the 

 internodes, and thus these horizontal lateral branches exhibit a form 

 which is like that of a many-times pinnate leaf possessing a dorsiventral 

 construction 1 . 



The intensity and the direction of the light on the one hand, and on 

 the other hand the relationships of correlation, play an important part 

 in bringing about the difference in direction of the several shoot-systems 

 of such trees as are built up out of plagiotropous shoots. The chief 

 shoots of the beech, for example, growing in the open are erect, in 

 feebler illumination, for instance in dense woods, they become almost 

 horizontal. Further investigation is required to determine the extent to 

 which the lateral shoots may be induced through the influence of the 

 more erect-growing chief shoot to assume a greater inclination to the 

 horizontal plane. 



Exceptional cases are not unknown. Vicia Faba, for example, is 

 a leguminous plant which, like many others of its order, is dorsiventral 

 in its branching 2 , its shoots are however orthotropous. Still in the largest 

 number of cases the rule above mentioned holds good. 



It may be of interest to quote from the lower plants some illustrations 

 of the relationship between symmetry and direction. 



In lichens the difference between dorsiventral thalli which are usually pressed 

 close to their substratum, as in the ' foliose ' forms, and the radially constructed 

 ones which usually stand erect or hang pendant, as in the ' fruticulose ' forms, is 

 very clear, and the transition from dorsiventral to radial organs which takes 

 place in many lichens is specially interesting 3 . Such a transition may take place 

 in three distinct ways : 



1. By convolution of a dorsiventral thallus or portion of a thallus. 



2. By formation of orthotropous vegetative outgrowths on a dorsiventral thallus. 



3. By development of the stalk of the fructification ; this becomes very con- 

 spicuous and leads to the production of peculiar vegetative organs if at a later 

 period of development the formation of the sporiferous portion is reduced or 

 suppressed. 



The following are examples of these three cases : 



1 It may not perhaps be superfluous here to point out that the relationships above shortly 

 mentioned are frequently incorrectly stated, because the relationships of position are examined not in 

 the fo/but in the unfolded leaves. Thus, for example, Wigand (Der Baum, p. 161) was mistaken 

 in saying that the lateral buds of the lime do not begin with the one-half position, having been 

 misled by the torsion of the internodes on the horizontal lateral branches, and having overlooked the 

 fact that the planes of symmetry of the lateral shoot and chief shoot do not coincide from the first. 

 The bud-diagram also which Frank gives (Die natiirliche wagerechte Richtung von Pflanzenteilen, 

 Leipzig, 1870, Fig. i) is not quite right with respect to this point. It is wrong in the position 

 of the axillary buds to the bracts and the position of the first leaves of the bud. 



- The inflorescences, like those of Vicia Cracca, are all turned to one side, the vegetative buds to 

 the other. Compare Figs. 78, 79. 



3 With reference to this see Reinke, Abhandl. liber Flechten IV, in Pringsh. Jahrb. xxviii. p. 191. 



