440 PHYSIOLOGY OF GROWTH. 



soil of the box with its root directed perfectly horizontally along" 

 the glass, while its fellow is placed with its root pointing verti- 

 cally downwards. After twelve to sixteen hours (20 C.), we 

 measure the growth in the straight and geotropically curved roots 

 respectively, in the latter case making use of the sheet of mica, 

 and determining not only the growth of the convex, but also that 

 of the concave (lower) side of the root. If, e.g., the growth on the 

 convex side of the root = 10 mm., that on the concave side -6 mm., 

 the growth along the median line (axis of the root) amounts to 

 8 mm. The straight, control root may have lengthened by 9' 5 mm. 

 Thence it follows, as Sachs first made clear, that the convex side 

 of curving roots grows somewhat more rapidly than a straight 

 root, under similar conditions. The concave side of the curved 

 root grows considerably more slowly than the straight root, and 

 the total growth of the former is notably less than that of the 

 latter. 



In researches on the growth of the main roots of Phaseolusor 

 Vicia Faba, we should not omit to make ourselves acquainted in 

 a general way with the form of the geotropic curvature which the 

 roots have experienced. For this purpose we provide ourselves 

 with the thin sheet of mica on which a series of concentric circles 

 has been scratched with compasses. By laying the sheet of mica 

 over the glass surface against which the root is growing, we can 

 readily determine the form of the root curve. At the commence- 

 ment of the geotropic nutation, it coincides with a circle of con- 

 siderable radius. Later the root curve appears less flat than in 

 the first stage of observation. Later still the curve no longer 

 coincides with any of the circles, but becomes parabolic. The 

 zone of most active growth is strongly curved ; in front of this 

 region and behind it the curve is much flatter. 



We will now proceed to consider the geotropic behaviour of 

 secondary roots, confining ourselves to the lateral roots of the 

 first and second order given off by the main roots of Phaseolus, 

 Pisum, Vicia, and Zea, Seedlings are cultivated in our culture 

 case behind a sheet of glass. The main root grows straight 

 downwards. The lateral roots of the first order, forming in 

 acropetal succession on the main root, do not take this direc- 

 tion, but grow, as is indicated in Fig. 142, more or less obliquely 

 downwards. We can easily satisfy ourselves that the lateral 

 roots of the first order do actually behave geotropically, by in- 

 verting the case, so that the main root is directed upwards. We 



