THE LTFE-HISTOEY OF TLANTS. 



87 



movement. We allude to the property of sensi- 

 tiveness. 



When an organ is irritated by any cause and 

 movement occurs as the consequence, the part is said 

 to he sensitive. The tip of the root has been proved 

 to possess this sensibility to iinpressions. A slight 

 touch, the application of 

 caustic, or the removal 

 of a thin slice from one 

 side, have all been 

 proved to be productive 

 of movement. But this 

 sensibility is confined to 

 the tip of the radicle for 

 a very minute space, the 

 consequent movement 

 taking place at some 

 little distance from the 

 point of irritation. Thus, 

 if the tip of the radicle 

 be irritated, the portion 

 bends away from the side 

 which has been irritated. 

 These curvatures are 

 only manifested when 

 the roots are growing 

 under natural conditions. 

 If the soil be too hot, or 

 too cold, the movements 

 are not observed. The 

 curvatures resulting 

 from contact are not 

 always observable at 

 once, but only 

 after the expi- 

 ration of some 

 hours. As the 

 tip, says Dar- 

 win, " is always 

 endeavouring to 

 bend to all sides, 

 it will press on 

 all sides, and 

 will thus be able 

 to discriminate 

 between the 

 harder and 

 softer adjoin- 

 ing surfaces. 



. . . Consequently, it will tend to bend from 

 the harder soil, and it wiU thus follow the lines 

 of least resistance. So it will be it it meet with a 

 stone, or the root of another plant in the soU. . . . 

 If the tip were not sensitive, and if it did not excite 

 the upper part of the root to bend away, whenever 

 it encountered at right angles some obstacle in the 



Fig. 13.— Knight's Wheel, showing the effect of growing 

 Seeds on the surface o( a vertically rotating "Wheel. 



Fig. 14. Effect of growing Plants on a horizontally rotating Wheel. 



ground, it would be liable to be doubled up in a 

 contorted mass." 



Above the apex, the radicle is also sensitive, but in 

 this case the effect is to cause the radicle to bend, 

 not away from, but towards the obstacle, so that it 

 curls round it and gets on the other side of it. 



In endeavouring to 

 give an idea of what 

 goes on in the tip of the 

 root, and of the way in 

 which it is thereby en- 

 abled to penetrate the 

 ground, we may avail 

 ourselves of the com- 

 parison made by Darwin 

 between the snout of a 

 mole or other burrowing 

 animal and the tip of 

 the root. By continual 

 movement of the head, 

 the animal feels any 

 stone or other obstacle, 

 as well as differences in 

 the degree of compact- 

 ness of the soil, and he 

 will turn away from the 

 harder side to that which 

 is softer and moister. 



The Caulicle.— 



While the radicle is 

 pursuing its downward 

 course provision is being 

 made for the 

 upraising of the 

 seed-leaves, or 

 "cotyledons," 

 and of the bud 

 or " plumule," 

 which is des- 

 tined to leng- 

 then into the 

 stem. Very 

 frequently, but 

 not universally, 

 there is a little 

 stalk interven- 

 ing between the 

 radicle and the 

 seed-leaves; or it may more correctly be said that 

 it always exists, but is sometimes so small as not 

 readily to be visible. This stalk is the " tigeUum," 

 or " caulicle." It constitutes the whit>e stalk which 

 supports the seed-leaves of Mustard and Cress, 

 and is particularly well seen in the French Bean 

 or in the Melon (Figs. 15, 16, 17). It may be 



