INFLUENCE OF GRAVITATION ON GROWTH. 763 



the most from 2 to 3 mm. from the apex, and the strongest curvature is therefore at 

 this spot, or very near the apex ; and when the organ lies in a horizontal position 

 the curvature is very strong, or the radius only very small (a few millimetres). It is 

 easy to see that when a very strong curvature takes place near the apex of a root, it 

 serves to fix it in the ground; while it is mechanically useful for the erection of 

 stems that they curve in larger flatter curves. In the jointed haulms of Grasses the 

 work of flexion is distributed over two or three nodes, a portion of the curvature 

 taking place in each node until the haulm again stands erect. 



Knight, the discoverer of the fact that gravitation is the cause of geotropic curvature, 

 thought that the curving upwards of the stem was occasioned by the food-materials 

 collecting in greater quantities on the under side and hence causing a more powerful 

 growth. Hofmeister, who called attention to the relation of the tension of the tissues to 

 the various curvatures of the parts of plants, explained the action of gravitation in caus- 

 ing an upward curving in the first place by an increase of the extensibility of the tissue 

 on the under side, which is in a state of passive tension. I have, on my part, directed 

 attention to the fact that the growth of the under side of organs placed horizontally 

 which have a tendency to curve upwards is accelerated, while that of the upper side is 

 retarded ; but whether this is caused by a corresponding distribution of the food- 

 materials, or by a change in the extensibility of the passive layers, or in any other way, 

 1 leave for the present undecided. 



The curving downwards of the roots of seedlings was explained by Knight In an 

 unsatisfactory way as a result of the softness and flexibility of the growing apex, a view 

 which was adopted by Hofmeister in a less crude form, and for some time also by myself. 

 It was assujued on this theory that the tissue of growing roots may be compared to a 

 tough piece of dough, which tends, from the force of its own weight, to curve down- 

 wards at the free unsupported end. I thought that by the excess of weight of the free 

 apex a traction was exerted on the growing cell-walls of the parts of the upper side which 

 curve, by which growth or deposition of food-material is promoted on this side, while 

 the reverse must be the case on the under side ; and I think that Hofmeister explained 

 the process in a similar manner. Frank therefore did not hit the nail on the head when 

 he merely insisted that the downward curving of the apex of the root depends on 

 growth being stronger on the upper side ; this we had admitted. It would have been 

 more to the purpose had he said why growth is more rapid on the upper than on the 

 under side of the apex of a root placed horizontal. Frank, on the other hand, was right in 

 maintaining that our explanation was untenable, because, as Johnson had already shown, 

 the apex of a root turns downwards even when its own weight is counterbalanced by an 

 equal or slightly greater one, and because the root, even when it rests on a horizontal 

 solid support, shows the same phenomena of growth which cause its apex to point down- 

 wards. The statements of Frank and the subsequent ones of Miiller were however 

 inadequate on the points in question. If I relinquish Hofmeister's view, which I had 

 previously in the main adopted, it is in consequence of more comprehensive experiments 

 on the growth of roots, and especially on their geotropic curvature. It would carry us 

 too far here to give the reasons for and against the theories which have been alluded to ; 

 and it would serve as little purpose to go into an explanation of particular phenomena, as 

 for example the fact that roots penetrate to a depth of from 2 to 3 mm. into mercury, 

 whether they impinge upon it vertically or obliquely \ 



It seems to me that any theory of geotropism can only be adequate if it is able to 



1 See Pinot u. Mulder, Ann. des Sci. Nat. 1829, vol. XVII, p. 94, and Bydiagen for de natuur- 

 kund. Wetensch. 1829, vol. VI, p. 429 ; also SpeschenefT (Bot. Zeitg. 1870, No. 5), whose statements 

 I am able lo confirm in tlie main by a number of experiments of my own. 



