yo6 



LECTURE XL. 



dorsal surface of the organ is everywhere turned outwards, the rolled up organ 

 constitutes a radially, constructed body, and certainly the most elegant confirmation 

 of the above statements lies in the fact that the dorsi-ventral organ in this rolled up 

 condition, where it has only incidentally become radial, now reacts also towards 

 gravitation and light as an orthotropic organ. Figure 398 will serve to show what is 

 meant. C represents the transverse section of a lamina in its extended condition, 

 and the arrows indicate the direction in which the structure changes, proceeding from 

 the lower to the upper side. If we now suppose this rolled together either as in A 

 or £, it is easy to see, and is indicated by the arrows, that what was previously the 

 lower side now runs round the rolled up organ as an external layer, and that the 

 dorsi-ventral structure has passed over into a radial one, which now confers on 

 the rolled up organ the chajracter of an orthotropic one towards gravitation and 

 light. 



It is not practically easy, either in the case of growing leaves or of the flat 



FIG. 358.— (See the text.) 



shoots of Liverworts, artificially to roll up the already extended organ in the 

 manner indicated, so that it will go on growing undisturbed, but the converse 

 change also demonstrates what I wish to prove. The subsequently flat leaves of 

 very many plants— e. g. of all Grasses, most Liliacese and other Monocotyledons, 

 and even those of many Dicotyledons, as the Water-lilies \Nuphar, NymphcBa), 

 Pitiguicula, and many others— are in the bud-state rolled together as in Fig., 398 

 A and B, either each individual leaf by itself {B), or so that the young leaves 

 envelope one another (^); on subsequent growth the rolled-up margins are drawn 

 apart, and the leaves become extended flat, as in C. Now so long as the young 

 leaves remain in the rolled-up condition, in the bud-state, as in A and B, they 

 are orthotropic, because they constitute a radial convolute structure; but as soon as 

 they have extended themselves, they become plagiotropic, and are placed obliquely 

 or horizontally with reference to gravitation, and extended at right-angles with respect 

 to the rays of light. 



That this is not simply a special peculiarity of a certain class of plants 



