«44 
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 k i k in of seedlings ^5^^^ 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 
maritimus), which are not bilaterally organised, tend to maintain a horizontal direction of growth ; 
they are therefore not geotropic] 
