410 



XXX. THE SEED OF ANGIOSPERMS. 



piece of elder-pith, and, after drying, gum and seed can be cut 

 together. 



The sections, in whichever way prepared, should be examined 

 in glycerine, as water makes the embryo swell, and come out 

 of the spermoderm. The embryo (Fig. 149, A), tills the entire 

 seed ; it is bent at its mid-length, so that the cotyledons (c) 

 lie alongside the hypocotyl (h), i.e., are incumbent. (Compare 

 the figure.) This disposition is characteristic of several tribes 

 of the Cruciferae, by some systematists collected into the section 

 Notorhizeae, and may be represented by the sign I I O. Another 

 characteristic method of folding of the embryo in Cruciferae, is 

 where the hypocotyl is folded over and applied to the edges of the 

 cotyledons. This is called accumbent, and may be expressed by 



the sign ~ O. They may 

 likewise be represented by 

 C C H and H respectively. 

 If the section is delicate and 

 has cut the seed perfectly in 

 the centre (as in the adjoining 

 Fig. A), we see at the base 

 of and between the coty- 

 ledons the small growing apex 

 (punctum vegetationis) of the 

 stem, the plumule, and can 

 also see, at the lower end of 

 the hypocotyl, the axis closed 

 by the radicle, covered by 

 a root-cap only a few cells 

 thick. The endosperm in 

 this seed is represented only by a single layer of aleurone-containing 

 cells. The grains which they contain take a yellow colour with 

 iodine, and thus betray their nature. The aleurone layer im- 

 mediately surrounds the embryo, and is itself surrounded by the 

 spermoderm, the so-called tes'ta. If we use a somewhat higher 

 magnifying power we can determine that this spermoderm (Fig. 

 149, B) consists of three layers of cells ; an innermost mem- 

 branous layer (*) owing its origin to the crushed inner integument ; 

 a second layer (c) the cell- walls of which are coloured yellow, and 

 towards the inner side are very strongly thickened ; and an outer- 

 most layer (e) of cells, which appears in concentrated glycerine as 

 a colourless, apparently homogeneous membrane, while its cells 



ft \c 



* 



FIG. 149. Capsdla Bursa-pastoris. A, 

 longitudinal section through a ripe seed ; h, 

 hypocotyl ; c, cotyledons ; v, vascular bundle 

 of the funicle (x 26). B, part of a longi- 

 tudinal section through the spermoderm and 

 the aleurone layer, after the action of water ; 

 e , the swollen epidermis ; c, the yellow 

 strongly-thickened layer; * the crushed 

 layer of cells ; a, the aleurone layer (x 240). 



