764 



MECHANICAL LAWS OF GROWTH. 



explain equally the positive and the negative descriptions, and to show why the same 

 external cause produces opposite results in cells and organs of precisely similar structure, 

 acceleration or retardation of growth on the under side and the reverse on the upper 

 side. 



If a number of organs grow in a horizontal or oblique direction without curving 

 either upwards or downwards, this may result from their not being geotropic and grow- 

 ing straight forward in the direction of their first origin, as rootlets of a high order 

 which grow downwards from the under side of their parent root, upwards from the 

 upper side, horizontally from the sides, or continue to grow straight and oblique 

 according to the direction of the primary root. To this must be referred, among other 

 phenomena, the striking one described by me that plants which grow in uniformly moist 

 soil emit a large number of fine roots out of it with their apices pointing upwards ; 

 there are even rootlets of the first or second order which spring from the upper side of 



Fig. 453. — Apparatus to illustrate the mode in which the geotropism of the roots h i k m of seedlings^ g g is overcome 

 when they come into contact with a moist surface. 



horizontal or oblique parent roots and grow straight upwards without being geotropic. 

 If the air is able to enter the ground freely, its surface is often dry, and the fine roots 

 which are directed upwards die off, as I have ascertained by growing plants in glass 

 vessels filled with earth. 



But even geotropic organs may grow obliquely or horizontally when other causes 

 oppose or counterbalance their geotropism. One of the most common of these causes 

 is the bilateral structure which makes an organ grow more strongly on one side from 

 internal causes. Since I shall recur to this subject in the next section, only a single 

 example need be given here. In the case of seedlings, rootlets of the first order not 

 unfrequently appear above the surface of the soil obliquely when it is uniformly moist ; 

 and I have convinced myself that this is the result in cases which have been observed 

 {e. g. Victa Faba) of a stronger growth of their lower side altogether independent of 

 geotropism, in consequence of which they always grow in a flat curve concave upwards. 

 But external causes may also act in opposition to geotropism even when this is very 

 strongly developed. Thus Knight and Johnson have shown, as I have recently described 



