MOVEMENTS OF IRRITATION. 507 



shoots are produced, while at the morphological base especially 

 numerous and vigorous roots are formed. The experiments with 

 inverted pieces of stem thus teach that the action of gravity 

 cannot be the immediate cause of the phenomena we have observed 

 in connection with the development of shoots and roots. 



It is certain that the polarity of plant structures, i.e. the 

 differentiation into base and apex, is not due to mysterious vital 

 forces, but that it owes its origin to external forces. We have 

 every reason to suppose that gravity was, in the first instance, 

 the chief cause of that polarity, and we may. form for ourselves 

 an idea somewhat as follows of the problem before us. When 

 gravity always acts in the same direction and for numberless 

 generations on plants, an inheritable peculiarity may finally 

 result, owing to summation of effects, which, in fact, we desig- 

 nate as polarity. Polarity would accordingly be regarded as an 

 after-effect phenomenon, induced by gravitation and stretching 

 beyond the life of the individual, as a phenomenon of inherent or 

 stable induction, or, as we are accustomed to say, as an inherit- 

 able disposition. From this point of view it would also be intel- 

 ligible that gravity is unable to exert any essential direct influence 

 on the development of buds and roots in structures having well- 

 marked polarity.* But gravity does nevertheless exert, under 

 particular circumstances, a striking influence on them, as we may 

 easily satisfy ourselves by an experiment, which I conducted as 

 follows. A fairly large zinc box was half filled with water. 

 Over the surface of the water were then arranged horizontally a 

 number of pieces of willow stem, 200 ram. in length and 12 mm. 

 in diameter, as can easily be effected by supporting them at their 

 ends on suitable supports projecting out of the water. The zinc 

 box I closed not completely air-tight with a cover, so that the 

 research material was in darkness. The experiments, which were 

 made in March and April, showed that the buds developed pre- 

 ponderatingly at the morphological apex of the pieces of stem, 

 while the roots formed chiefly at the base. It is, however, of 

 special significance for us here that the former developed chiefly 

 on the upper, the latter on the lower side of the horizontally-lying 

 stems, a result without any doubt dependent on the action of 

 gravity. In investigations of this kind, a number of pieces of 



* All shoots do not exhibit such decided polarity as willow twigs, and such 

 are then accessible also to local induction. See Sachs, Lectures on Plant 

 Physiology. 



