ROLLED UP PLAGIOTROPIC ORGANS ARE ORTllOTROPIC. 



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

 of the large flat Lichen Pelligera canina. The organisation of the vegetative bod)-, 

 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 {r r, Fig. 399), rise up vertically, because 

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

 at r r 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 hlandica), 

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

 a w^ay 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 pyxifa/a, 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, flat, 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- 

 fashioned champagne-glass and circular in section : these 

 are strictly orthotropic. 



It will be seen from the foregoing that the clear 

 understanding of the relations between radial or dorsi- 

 ventral structure, on the one hand, and orthotropic 

 or plagiotropic growth on the other, involves ideas 

 which present much that is extraordinary and difficult, 

 and I confess that it was only after many years of 

 thought that I was able correctly to apprehend the 

 matter; it must be 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 heHotropism, 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 Faha, 

 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 cftect-; the 

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

 of the apparatus represented in Fig. 384, p. 682, I con\-inced myself in the 

 first place that the lateral roots are verv cli-arly affected by centrifugal force. 



FIG. 399.— .4 a fructification (Apo- 

 thecium) a of the Lichen Peltigira 

 canina on the rolled up supportinfr 

 portion r r, which springs from the 

 flat vegetative body t. B transverse 

 section of A. 



