THE EMBRYO AND THE SEED 317 



as a permanent nourishing tissue, which acts till the embryo-sac is well 

 advanced. 



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. 235), 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 

 (m.h.) passes through the micropyle, and traverses the funicle towards the 

 same source of supply. 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 

 among the 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 (Fig. 239). It is 

 probably a relatively primitive state, and it is FlG - 2 3 6 - 



. . f ... 1 t-> 1 Vertical section through a 



found in such families as the Kanunculaceae Peppercorn. «m= embryo. 



,,, .. , . .,» ,1 « = endosperm. * = perisperm. 



andMagnohaceae, and in mostMonocotyledons. The testa is shaded. ^r= P eri- 



nr 11 /— i_ j r j-l • carp. (After Baillon.) 



Moreover, all Gymnosperms have seeds 01 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. 239). 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. 237). 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 perispenn, and it is found in the Peppercorn (Fig. 236), 

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

 albuminous seed is morphological rather than physiological. But 

 in the great majority of cases the nucellus is obliterated early, owing 



