ACTION OF GRAVITATION ON GROWTH IN LENGTH, 



853 



folds are found upon the concave under surface in consequence of compression, just 

 as in the nodes of Grasses. When the root is curving geotropically, all the cells 

 within the curving portion usually grow, but their growth is slower the nearer they 

 are to the lower surface which is becoming concave. In passing from the convex 

 surface, where the cells are fully developed and contain much sap, to the concave surface, 

 where the cells present the appearance of young undeveloped cells containing much 

 protoplasm, all intermediate forms may be met with. Since, therefore, the development 

 of the cells of the under side is very considerably impeded, it is possible for those 

 of the upper side to undergo a more or less excessive elongation. Some observations, 

 as yet incomplete, seem to indicate that the retardation of 

 the growth in length of the under side is accompanied by a . 



more vigorous growth of the cells in a radial direction, and 

 the acceleration of that of the upper surface by a less vigorous 

 radial growth. 



If thick primary roots be split and be treated in the manner 

 described with reference to stems, the same phenomena 

 (though in exactly the opposite direction) are generally pro- 

 duced : this shows that geotropism is not merely a property 

 of the root as a whole, but also of each of its constituent 

 parts. These observations are, however, very difficult to carry 

 out. A persistent effect of the commencing geotropic action, 

 which was so well-marked in the case of stems, is stated 

 to occur in roots also by Ciesielski and Frank. I have not yet 

 succeeded in detecting it, but I will not reject the fact, for 

 other methods will perhaps afford more satisfactory results. 



4. The chief result of the observations which I have made 

 hitherto is, I believe, this, that the phenomena of upward 

 geotropic curvature are essentially the same, though taking 

 place in the opposite direction, as those of downward geotropic 

 curvature, and that therefore the mechanical explanation of the 

 one will include that of the other. This necessarily implies 

 the incorrectness of the older explanations offered by Knight 

 and by Hofmeister. 



Knight, the discoverer of the fact that it is gravitation 

 which induces geotropic curvature, believed the upward cur- 

 vature of the stem to be due to an accumulation of nutrient 

 materials towards the lower surface, which would induce 

 more vigorous growth. Hofmeister, who regarded the state 

 of tension of the tissues to be the most important factor in 

 producing the curvatures of parts of plants, considered that 

 the action of gravitation in producing an upward curvature 

 was to increase the extensibility of the passively stretched 

 tissues of the lower side. I pointed out that the growth of 

 the under surface of an organ capable of curving upwards 

 was accelerated, and that of the upper surface retarded : I 



did not at the time express an opinion as to whether these modifications of growth 

 were due to an altered distribution of plastic material or to a change in the extensibility 

 of the passive layers of tissue. 



Knight explained the downward curvature of primary roots in a somewhat obscure 

 manner by referring it to the softness and flexibility of the growing apex, a view 

 which was adopted by Hofmeister in a more precise and logically complete form, 

 and one which I for a time accepted. It was assumed that the tissue of the growing 

 root was comparable to soft dough, and that the unsupported end tended to curve 

 downwards under the influence of its own weight. I considered that the weight of 



Fig. 484.— The growing and 

 curving end of the primary root of 

 Vtcia Faba placed horizontally in 

 loose earth behind a thin lamina 

 of talc. The root was marked out 

 into five portions, each two milli- 

 metres in length, beginning from 

 the punctiim vegetationis, A tri- 

 angular index of paper was fixed 

 to the talc so as to correspond to 

 the mark on the root ; by means 

 of this the change of position of 

 the marks could be detected. A 

 at the commencement of the ex- 

 periment, B after one hour, C at 

 the end of two hours, D after seven 

 hours, E after twenty-three hours. . 



