392 EFFECT OF GRAVITY UPON GROWTH [Cn. XV 



growing organ as it might upon any other heavy body. The 

 second is a vital effect, having no immediate, direct, physical 

 relation to the cause. The first may be called false geotro- 

 pism; the second, true geotropism. 



1. False geotropism may be treated very briefly, since it is 

 not a true growth phenomenon. As an example of such geotro- 

 pism may be cited the downward bending of the top of a stem 

 heavily laden with a head of seed or with fruit, or the upward 

 growth of the long fronds of kelp in the sea on account of the 

 buoyant effect of the dense sea water. In such cases the 

 direction of the growth can be accounted for upon well-known 

 principles of hydrostatics. 



2. True Geotropism. - - The biological effect, on the other 

 hand, which is seen in true geotropism is often opposed to 

 the gross physical one. It shows itself especially in various 

 groups of sessile plants and animals. Since, unfortunately, com- 

 paratively few experiments have been made upon geotropism 

 in animals, the great mass of our knowledge on this subject is 

 derived from studies on plants. 



The general fact of geotropism strongly impresses one w T ho 

 stands on the shore of a lake in our northern country and looks 

 across to the dense pine forest on the opposite side. The 

 landscape is composed of vertical and horizontal lines 

 vertical lines below formed of close-set trunks of trees, hori- 

 zontal lines above formed of the great branches. If at one side 

 a steep slope ascends, its outline is obscured by the grill-work 

 of perpendicular lines formed by the vegetation which clothes 

 it. Here the effect the dissimilar effect of gravity in 

 determining the direction of growth of two organs, trunk and 

 branch, is seen. There are, however, still other organs which 

 may respond to the same stimulus. Among these are the 

 roots, flower stalks, and leaves of phanerogams ; the vertical 

 parts of many vascular cryptogams ; the sporiferous hyphse of 

 fungi and some vertical algse, e.g. Chara. There is no need to 

 examine all these cases of geotropism, but only such of them as 

 will help us to get at the principles of the action of gravity in 

 determining the direction of growth. 



a. Hoots.'- -When a seedling is so placed that its radicle is 

 horizontal, the radicle does not continue to grow out in the 



