ROLLED UP PLAGIOTROPIC ORGANS ARE ORTHOTROPIC. JO] 



is shown by reference to Fig. 399, which represents the fertile marginal lobes 

 of the large flat Lichen Peliigera canina. The organisation of the vegetative body, 

 which grows on the flat surface of the soil in woods, is sharply dorsi-ventral ; 

 it is green and smooth on the upper side, colourless and furnished with roots 

 on the lower (cf. Fig. 249, p. 391), and in consequence of this dorsi-ventral structure 

 it is closely appressed to the horizontal surface. Of the lobes at the margin, those 

 which bear the fructifications or apothecia, a {rr, Fig. 399), rise up vertically, because 

 this part of the otherwise flat vegetative body becomes rolled up, as shown 

 at r7- in the figure. In this case it is the upper side which comes to be external 

 on the rolling up : in the Lichen called ' Iceland Moss ' (Cetraria Islandicd), 

 which consists of branched ribband-like shoots, the inrolling takes place in such 

 a way that the organic under or ventral side comes to be external, and in this 

 case also the flat dorsi-ventral structure gives rise to a radial one, which, for 

 that reason, is orthotropic. Yet another Lichen, Cladonia pyxilata, well known 

 from its elegant appearance, may also serve for the de- 

 monstration of what has been said above. It has two 

 forms of shoots : the exclusively vegetative shoots are 

 thin, fiat, and dorsi-ventral, and therefore lie closely on 

 a horizontal substratum ; from these spring the shoots 

 of the second form, structures shaped like a tall old- 

 feshioned champagne-glass and circular in section : these 

 at-e strictly orthotropic. 



It will be seen from the foregoing that the clear 

 understanding of the relations between radial or dorsi- 

 ventral structure, oh the one hand, and orthotropic 

 or plagfotropic growth on the other, involves ideas 



- FIG. 399.—.^ a fructification (Apo- 



which present much that is extraordinary and difficult, thedum) » of me uchen /=j*^i™ 



caitina on tlie rolled up supporting 



and I confess that it was onh' after many years of portion ^r.wiiicii springs from the 



flat vegetative l^ody A B transverse 



thought that I was able correctly to apprehend the section or .4. 



matter; it must b6 added that even now several points 



still await investigation. This is especially true of the fact that the orthotropic 



or plagiotropic growth in some cases is due exclusively to gravitation, in others 



to the co-operation of geotropism and heUotropism, and in others again the so-called 



hydrotropism co-operates. 



I will give one or two examples of each of these cases.- 



I demonstrated in 1874 that the lateral roots which originate from primary roots 

 assume their oblique direction solely under the influence of geotropism, and this 

 was the first case of geotropism of this kind which was known at all. Suppose 

 Fig. 400 to represent the upper part of the primary root of a seedling of Vicia Faba, 

 which has been grown behind a pane of glass in a box filled with loose soil, and 

 has developed numerous lateral roots, which have grown straight out laterally and 

 downwards at oblique angles with respect to the horizon. At that time it was 

 doubtful whether the latter happened in consequence of a geotropic effect ; the 

 question therefore was, whether lateral roots are geotropic at all. By means 

 of the apparatus represented in Fig. 384, p. 682, I convinced myself in the 

 first place that the lateral roots are very clearly affected by centrifugal force, 



