844 



MECHANICS OF GROWTH. 



growing straight forward in the direction of their first origin, just as rootlets of a 

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

 the upper side, horizontally from the vertical 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 ; these are rootlets of the first or second order which spring from the upper 

 side of 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. 



Fig. 483.— Apparatus to illustrate the mode in which the geotropism of the roots h i k m of seedlings ^^^ is overcome 

 when they come into relation with a moist surface (hydrotropism). 



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 organisation 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. Vicia Faha) of a stronger growth of their lower 

 side altogether independent of geotropism, in consequence of which they always 



maritimns), which are not bilaterally organised, tend to maintain a horizontal direction of growth ; 

 they are therefore not geotropic] 



