THE EMBRYO AND THE SEED 



281 



In addition to this the embryo-sac itself may frequently put out local 

 HAUSTORIA, which penetrate to favourable sources of nourishment. A good 

 example of this is seen in Rhinanthus (Fig. 222), where, in addition to the 

 epithelial jacket (n.j.), haustoria are formed at both ends of the sac. The 

 chalazal end (c.h.) extends so as to reach a mass of nutritive tissue (nt.) close to 

 the end of the vascular strand. From the micropylar end a similar haustorium 

 passes through the micropyle, and traverses the funicle towards the same 

 source of supply (m.h.). The haustorial connections may be still more 

 elaborate in other plants (Plantago). Such arrangements indicate the impor- 

 tance of the nourishment of the sac. especially in its earlier stages. They 

 also provide interesting analogies with the behaviour of parasites, whether in 

 Fungi or in Flowering Plants. 



The function of the endosperm is to provide temporary nourishment 

 for the embryo which it surrounds. But the amount of the supply, 

 and the time when it is yielded to the embryo 

 may vary. Two main types of seed arise 

 accordingly. In the first the embryo grows 

 slowly, and keeps in close touch with the 

 endosperm, -which remains relatively large 

 till the seed is ripe ; it embeds the embryo 

 and is stored with food. The result of 

 this is the " albuminous " seed (p. 10). It is 

 probably a relatively primitive state, and it 

 is found in such families as the Ranunculaceae 

 and Magnoliaceae, and in most Monocotyledons. 

 Moreover, all Gymnosperms have seeds of this 

 type. In the second the embryo develops more quickly. It absorbs the 

 available nourishment early, so that at ripeness little or nothing remains 

 of the endosperm. Its function has been temporary. Such seeds are 

 called " ex-albuminous " (Fig. 226). Intermediate states are found, as 

 in the Leguminosae, which, though usually held to be exalbuminous, 

 have in many cases a band of mucilaginous endosperm covering the 

 embryo (Fig. 224). While the substances stored in the endosperm 

 provide for the further growth of the germ, they also supply the 

 staple food of man in the various cereal grains. 



In some cases the store of food for the embryo may in part be out- 

 side the embryo-sac, in the chalazal region of the nucellus. Such tissue 

 is called perisperm, and it is found in the Peppercorn (Fig. 223), or 

 the seed of the Water-Lily . The difference from an ordinary albu- 

 linous seed is morphological rather than physiological. But in the 

 reat majority of cases the nucellus is obliterated early, owing tq 

 the precocious growth of the megaspore which it envelops while 



FIG. 223. 



Vertical section through a 

 Peppercorn. em= embryo. 

 e endosperm. p= perisperm. 

 The testa is shaded. per=peri- 

 carp. (After Baillon.) 



