852 MECHANICS OF GROWTH. 



been artificially bent until it was dislocated. These changes are produced by a con- 

 siderable shortening of the upper surface which accompanies the very vigorous growth 

 of the lower surface. That this is the case is shown by the following measurements 

 made on the nodes of Maize, the thickness of which in the plane of curvature was 

 from 10-12 mm. 



Length of the Node. 



Before curvature. After curvature!, 



4*3 mm. 2*5 mm. 



4*1 » 9*o „ 



4'o 5> 3*o » 



5-0 „ iro „ 



5*o )} 4'5 5, 



5'o » 12-5 „ 



Median longitudinal sections through the curved nodes showed that the cells of 

 the epidermis and of the subjacent tissue of the underside had undergone a cor- 

 responding elongation, but that they had not undergone division, whilst those of the 

 upper surface had not grown and had become so compressed by the curvature that 

 the above-mentioned folds of the tissue had been produced. 



3. The downward cur'vature of the tap-roots of seedlings'^ was studied especially in 

 a large-seeded variety of Vicia Faba, in Peas, Acorns, and Horse-Chestnuts. They 

 were placed horizontally either in moist air, or in water, or in damp earth. In the 

 last case, the seedlings were in a box having oblique walls of glass or of talc, which 

 permitted the observation of the roots during their growth and curvature. 



The statement that it is only those portions which are still growing that are 

 capable of curvature holds good also for roots. I showed, in opposition to earlier 

 views, that it is not one part only of the growing region but the whole of it, as in 

 the case of stems, which exhibits geotropism (Fig. 484). Since the whole growing 

 region (as was shown in Section 17) is only from 8 to 10 mm. long, and in many roots 

 even shorter, and since the curvature can only be considerable in the middle zones, 

 the curvature appears, especially after a considerable time, sudden and sharp with a 

 very small radius, a condition which is of considerable mechanical advantage to the 

 penetration of the roots into firm soil. Since the considerations which were stated 

 above with reference to stems may be generally applied with propriety to the curvature 

 of roots, the form of the curvature appears to be in this case, as in the former, only 

 at first that of the segment of a circle of large radius; but this is merely apparent, 

 for, since the apex of the root is directed downwards in consequence of the curvature, 

 the younger transverse zones are brought into a position which is unfavourable to 

 geotropism, whilst the oldest soon cease to grow and can therefore curve no further. 

 It is the zones which are in the middle phase of growth which undergo the greatest 

 curvature, for these not only grow rapidly and for a considerable time, but they have 

 also this advantage, that they do not at once come to occupy, in consequence of their 

 curvature, a position which is unfavourable to geotropism, as is the case with the 

 youngest zones. A more detailed account of the conditions which determine the form 

 of the curvature of roots which are either horizontal, oblique, or erect, will be found 

 in my paper which is here quoted. 



The measurement of the growth of the upper and under sides of roots is much 

 more difficult than in the case of stems and of the nodes of Grasses. I found that 

 the growth of the upper side was as vigorous, or even more so, as it was when the 

 root retained its normal position and form. The lower surface, however, is con- 

 siderably hindered in its growth, and. it appears from Ciesielski's statements, that 



^ The curvature took place in six days. 



2 Sachs, Arb. d. hot. Inst, Wurzburg, 1873, Heft III. 



