CHAP. II. ACTION OF THE BADICLE. 71 



when the radicle instead of travelling straight down 

 the glass made a semicircular bend; but Fig. 52 

 shows that this may occur when the track is rectilinear. 

 The apex by thus rising, was in one instance able to 

 surmount a bristle cemented across an inclined glass- 

 plate ; but slips of wood only ^ of an inch in thickness 

 always caused the radicles to bend rectangularly to 

 one side, so that the apex did not rise to this small 

 height in opposition to geotropism. 



In those cases in which radicles with attached fila- 

 ments were placed so as to stand up almost vertically, 

 they curved downwards through the action of geotro- 

 pism, circumnutating at the same time, and their 

 courses were consequently zigzag. Sometimes, how- 

 ever, they made great circular sweeps, the lines being 

 likewise zigzag. 



Radicles closely surrounded by earth, even when 

 this is thoroughly soaked and softened, may perhaps 

 be quite prevented from circumnutating. Yet we 

 should remember that the circumnutating sheath-like 

 cotyledons of Phalaris, the hypocotyls of Solanum, 

 and the epicotyls of Asparagus formed round them- 

 selves little circular cracks or furrows in a superficial 

 layer of damp argillaceous sand. They were also 

 able, as well as the hypocotyls of Brassica, to form 

 straight furrows in damp sand, whilst circumnutating 

 and bending towards a lateral light. In a future 

 chapter it will be shown that the rocking or circum- 

 nutating movement of the flower-heads of TrifoUum 

 subterraneum aids them in burying themselves. It is 

 therefore probable that the circumnutation of the tip 

 of the radicle aids it slightly in penetrating the 

 ground ; and it may be observed in several of the 

 previously given diagrams, that the movement is 

 more strongly pronounced in radicles when they first 



