FUNCTIONS OF THE SEED. 291 



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

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



9. FUNCTIONS OF THE SEED. 



605. 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 occur in the perispermic or albuminous seed of Euonymous, and 

 the aperispermic or exalbuminous seeds of most Cruciferae. 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, &c., and of laying up a store of nourishment 

 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 preserving 

 the vitality of the embryo. 



606. When seeds are matured, they are detached from the plant in 

 various ways. They separate from the funiculus at the hilum, and 

 remain free in the cavity of the pericap, 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 

 Cardamine. In the Geranium (fig. 455), the seed vessels are coiled 

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

 In the succulent fruit of Momordica Elaterium, or squirting Cucum- 

 ber, the cells vary in their size and contents in different parts; some 

 containing thick matter become distended at the expense of others 

 with thinner contents, and the force of endosmose ultimately causes 

 rupture of the valves at their weakest point, viz., where they are 

 united to the peduncle. When this takes place, the elasticity of the 

 valves sends out the seeds and fluid contents with great force through 

 the opening left by the separation of the peduncle. In the Impatiens 

 or Balsam, the seed-vessel opens with force by a similar process, the 

 five valves curving inwards in a spiral manner, in consequence of 

 the distension of the outer large cells. The seeds are discharged be- 

 fore they are dry. In the case of Mignonette (fig. 479), the seed- 

 vessel opens early, so as to expose the seeds ; and in Cuphea, the 

 placenta pierces the ovary and floral coverings early, so as to render 

 the seeds naked. 



