/ 



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 gotxl 

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

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

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

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

 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 earher 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 nourislniicnt 

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

 /.nd 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 ^''*^- --^■ 



^ . . , . .,. 1 T-» 1 V'ertical section through a 



is found m such iamilies as the KanunCUlaceae Peppercorn. <rm = embryo. 



1 -n T T 1 • T»T 11 e = endosperm. /) = perispenn. 



and IVlagnoliaceae, and m most Monocotyledons. The testa is shaded. pcr=pcr\- 

 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 arc 

 called " ex-aibuminous " (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 tlie 

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

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

 minous seed is morphological rather than physiological. But in tiie 

 great majority of cases the nucellus is obhterated early, owing to 

 the precocious growth of the megasporc which it envelops while 



