THE MOVEMENTS OF PLANTS. 



In either case the young stem issues from the seed-coat more or 

 less in the form of an arch. 



The radicle soon begins to " circumnutate," i.e. " bows 

 around," causing the tip of the root to move more or less in a 

 circular manner, enabling it to find a point of least resistance. 

 The apex of the root would seem to continue to grow with a 

 circular motion, as far as possible, beneath the soil, as is indicated 

 by the spirally-twisted condition which sometimes obtains 

 between two roots in close approximation. 



As long as the arching stem is below the soil this motion aids 

 the ascent by freeing it from the pressure of the earth. As soon 

 as it is free the concave side grows more rapidly than the convex, 

 so that it soon becomes straight. 



Hydrotropism. — This term, signifying "turning water- 

 wards," is given to roots, &c, which are influenced by the 

 presence of moisture, and are thereby induced to grow in the 

 direction of it. An experiment of Sachs' of growing beans, &c, 

 in a sieve full of damp materials, through the meshes of which the 

 roots protruded at first vertically, but then applied themselves to 

 the wet surface of the sieve again, proves that water more 

 powerfully influences the tips than does gravity. It is due to 

 this property that roots of trees have been known to grow to 

 great lengths to reach a distant water supply, the vapour coming 

 from the source having apparently induced the root to grow in 

 the direction of it. 



Phototropism and Heliotropism. — Turning " lightwards " 

 and " turning sunwards " are the terms indicating the well- 

 known property of plants being "drawn to a source of light." 

 The first term applies to the growth of stems upwards towards the 

 general illumination of the sky ; the second term to every case when 

 an organ is illuminated more in one direction than another. The 

 first is probably the original cause of the erect growth of stems ; 

 for an experiment made by Mohl * of growing plants in a box on 

 a perforated shelf and illuminated solely from below showed that 

 they now grew vertically downwards. Erect growth is generally 

 attributed to "negative geotropism," or " apo-geotropism," t as 

 being in the opposite direction to the action of gravity ; but there 



* The Vegetable Cell, p. 146. 



f It must be remembered that these and other like terms only express 

 observed facts, but in no way account for them. 



C 2 



