-10 LECTURE XL. 



by the diagram in Fig. 401. It represents one of the turf-blocks seen from the 

 left flank ; this presents a square outline, the body of the block being to a certain 

 extent drawn in perspective, so that the anterior surface turned towards the ray 

 of light L is to be supposed foreshortened to the observer. 



The conformity to law of the phenomena now comes out clearly. The 

 orthotropic stalks M and W of the supports of the sexual organs have placed 

 themselves tolerably exactly in the direction of the incident ray of light Z— 3 

 proof that their positive heliotropism must act far more strongly than their geo- 

 trppism. On the contrary, the flat vegetative shoots to the right above and to the 

 left below in the figure are placed nearly at right angles to the incident rays, and 

 also the flat shoots at h to the left above and /j would have exactly the same 

 direction— i.e. the direction h^, h^ and likewise i^, 4 — if they were not prevented by 

 the resistance of the solid turf. If possible these relations come out still more 

 clearly in the case of those flat shoots which are growing on the square flank of 

 the turf-block turned towards the observer, only it must be observed that the shoots 

 provided with the thick black lines stand out from the plane of the paper at 

 approximately a right angle. The root-hairs on the under side of the flat shoots 

 are also seen, indicated by thin parallel lines : they also are orthotropic, like the 

 supports M and W, but turned away from the source of light. 



After what has already been said of geotropisrn and heliotropism, the ortho-: 

 tropic organs of this plant need no further description ; as regards the plagiotropic 

 flat dorsi-ventral shoots, on the contrary, considerable space would be needed to 

 explain their behaviour. Referring the reader therefore tp my detailed treatise 

 published in 1879, I will only remark that the plagiotropic position of these 

 shoots results from the co-operation of their heliotropic and geotropic properties, and 

 perhaps the reader will most readily apprehend what is intended by supposirig one of 

 the root-hairs on the underside of the flat shoot to represent the primary root of an 

 ordinary seedling ; it is then to be further supposed that there is in connection 

 with this one rQOt-hair a very narrow piece of the tissue of that part of the flat 

 shoot which belongs to it, much as if it had been cut out by means of a cork- 

 bOrer. This piece of tissue would then correspond to, the shoot of a seedling 

 Phanerogam which had placed itself in the direction of the ray of incident light L. 

 Now it is easy to understand that the whole of the flat shoot of a Marchantia 

 with its root-hairs might be imagined as consisting of many thousands of 

 minute seedlings, laterally connected with one another; or, in other words, the 

 whole plagiotropic flat shoot consists fundamentally of nothing but orthotropic ele- 

 ments, which however are combined into a flat surface. However I must unfor- 

 tiunately avoid pursuing this very fruitful idea to its further consequences. 



The common Ivy {Hedera Helix) may be adduced as a second example of 

 plagiotropic growth under the simultaneous influence of different directive forces. 

 If a seedling or a rooted cutting of the Ivy grows in the neighbourhood of a wall 

 or a vertical surface of rock, the leaf-shoot applies itself closely to the surface of 

 the wall, &c., so that the leaves are situated to the right and left ; the free 

 anterior surface of the shoot-axis bears no leaves, and the surface applied to the 

 wall produces clinging roots which fix the shoot. Closely applied to the wall up 

 to the bud, then, the shoot grows up vertically,. The lateral . shoots which now 



