FUNCTIONS OF THE SEED. 343 



pericarp, the radicle may be directed upwards or downwards, or 

 laterally, as regards the ovary. In an orthotropal ovule, attached to 

 the base of the pericarp, it is superior (fig. 617). So also in a 

 suspended anatropal ovule, as in fig. 579, p. 329. In other anatropal 

 ovules, as in figs. 588, 600, 621, the radicle is inferior. When the ovule 

 is horizontal as regards the pericarp (fig. 619), the radicle, r, is either 

 centrifugal, when it points to the outer wall of the ovary; or 

 centripetal, when it points to the axis or inner wall of the ovary. 



9. — Functions of the Seed. 



The seed contains the embryo or germ, which, when placed in 

 favourable circumstances, is developed as a new plant. The embryo 

 is usually of a whitish or pale colour, resembling the perisperm when 

 present, and sometimes scarcely distinguishable from it at first sight. 

 Occasionally, however, it is of ' a greenish or yellow hue. Instances 

 of this occurs in the perispermie or albuminous seed of Euonymus, 

 and the aperispermic or exalbuminous seeds of most Cruciferse. The 

 changes which take place in the composition of the seed, and in its 

 coats, are with the view of protecting the embryo from vicissitudes 

 of temperature, moisture, etc., and of laying up a store of nourish- 

 ment for its after growth. The coats become thickened and hardened 

 by the deposition of lignine ; and in its interior, starch, nitrogenous 

 compounds, phosphates, and sulphates, besides oily and fatty matters, 

 various organic acids, tannin, and resins, are found. The specific 

 gravity of the seed is much increased, so that it usually sinks in water, 

 and it becomes more capable of resisting decomposition, and preserv- 

 iag the vitality of the embryo. In some instances where air is con- 

 tained in their envelopes seeds float iu water. 



When seeds are matm*ed, they are detached from the plant in 

 various ways. Th^ separate from the funiculus at the hilum, and 

 remain free in the cavity of the pericarp, which either falls along with 

 them, or opens in various ways so as to scatter them. The elasticity 

 with which some seed-vessels open during the process of desiccation 

 is very great. It may be seen in Hura crepitans, Common Broom, 

 and Oardam'ine. In the Geranium (fig. 551, p. 306) the seed-rvessels 

 are coiled upwards on the elongated beak, and in this way the seeds 

 are dropped. In the Cyclamen the peduncle curves towards the 

 earth so as to place the seed-vessels in a position suitable for germina- 

 tion. In the succulent frijit of Ecballium Elaterium, or squirting 

 Cucumber, the cells vary in their size and contents in difierent parts ; 

 and by the force of endosmose a rupture of the valves takes place at 

 their weakest points-=-viz., where they are united to the peduncle. 

 By the elasticity of the valves the seeds and fluid contents are sent 

 out with great force through the opening left by the separation of the 



