620 The Outline of Science 



is suggested that the root is in equilibrium only when these starch 

 grains rest on the true bottom wall ; that if they lie on a side wall 

 the difference is perceived, and the root curves till the tip points 

 downwards, the grains resume their proper position, and equili- 

 brium is restored. It is certain that some Crustaceans maintain 

 their balance by a similar apparatus of grains of sand resting in 

 the cavity of the ear. In the case of the root the experimental 

 investigation is beset with difficulties. But though the theory can- 

 not be regarded as definitely established it possesses considerable 

 probability, and it may be that in these special cells we have the 

 nearest approach that the plant shows to a sense organ. 



Why does the root curve down and the stem shoot up? To 

 that question we can as yet give no answer. We can only say that 

 it is their nature so to do. Many botanists expect, however, that 

 as our knowledge of the varying structure and composition of 

 living matter grows more precise, we shall be able to interpret 

 the matter in terms of physics and chemistry; the theories which 

 have been formed so far are unsubstantiated the imagination of 

 the scientist occasionally outruns his facts. 



6 



Tendrils 



The stem of the plant supports the leaves in the light, but in 

 many plants a weak stem bears the weight of the leaves by twining 

 round some stronger neighbour. The twining movement is of im- 

 mense importance to plants like our honeysuckle, hop, and bind- 

 weed, which are thus much superior to the bramble, rambling over 

 a stone wall secured only by its spines, or the goose-grass, ram- 

 bling over the bramble secured only by its hooks. In the great 

 forests of the Tropics a great proportion of the vegetation con- 

 sists of such types the lianes twining round trees, twining 

 round each other, forming an impenetrable confusion of coiled 

 and twisted cordage, reaching out to the light above the forest 

 roof, supported on the dead pillars of trees they have strangled. 



