438 THE BIOLOGY OF FLOWERING PLANTS 



fixation of the radicle the sheath-like cotyledon begins to 

 elongate ; it becomes sharply bent just behind the seed, 

 and this bent portion pushes through the soil dragging the 

 seed with it. In some species of the genus (Allium) a special 

 little sharp boring process is developed on the back of the 

 bend. The tip of the cotyledon remains in the seed, with- 

 drawing food from the endosperm, until it withers and the 

 seed falls off. Later the first foliage leaf bursts through 

 the side of the cotyledon. 



In the sunflower the elongation of the hypocotyl pulls 

 the cotyledons, protected by the seed coat, through the 

 soil. In the castor bean the elongating hypocotyl pulls the 

 cotyledons from their place between the halves of the 

 endosperm and carries them up. The part which actually 

 pushes through the soil is, in both cases, a sharply bent 

 portion of the hypocotyl ; the mustard and many others 

 behave similarly. In hypogeal seedlings it is of course the 

 elongation of the epicotyl which carries the plumule up. 

 The occurrence of a sharp bend is frequent here too, as in the 

 pea and the garden nasturtium. The importance of this 

 curvature is very great. If the plumule were erect the 

 earth would be forced against it and between the leaves, 

 separating them, increasing the resistance, and damaging 

 the most tender parts. Such curvatures are common not 

 only in seedlings but also in young shoots of perennial 

 herbs ; they are not, however, found on all shoots which 

 must penetrate the soil. They also occur on petioles, e.g. of 

 the wood anemone. In the dicotyledonous seedling 

 etiolation results in the more rapid elongation of the axis — 

 hypocotyl or epicotyl as the case may be — and in the leaves 

 remaining unexpanded. Here again the normal darkness 

 in the soil is responsible for a condition which aids penetra- 

 tion. The cause of the curvature of the axis or petiole does 

 not seem to be always the same. In general it is geotropic, 

 though the organ straightens out only, or more rapidly, in 

 the light. In some cases, however, an autonomic curvature 

 is present, and in some the mechanical resistance of the soil 

 is effective. The recent papers by Sperlich (191 2), Salisbury 



