426 XXXI. THE PKUIT. DEVELOPMENT OP THE FLOWER. 



bundles are scattered in the entire tissue. The five cells, forming 

 the " core," are covered each by a smooth, hard, cartilaginous 

 membrane, the endocarp. This corresponds with the shell of the 

 plum-stone. It consists of several layers of sclerenchyma fibres, 

 thickened almost to the obliteration of their cavity, and the 

 thickening layers of which are pierced by fine pores. Surface 

 sections show that these sclerenchyma fibres slope irregularly, 

 often are bent, and in the different layers have oppositely inclined 

 courses. The five cells often separate in the middle, forming a 

 central cavity, into which the individual cells then usually open. 

 At the base of each cell are inserted two ovules, of which both, or 

 only one, produce seeds, or of which often neither develops 

 further. 



The seed is almost filled by an embryo of the same structure 

 as in the plum or cherry. The brown spermoderm , on the other 

 hand, is much thicker than in the last-named plants. In cross- 

 section it shows an epidermis, the cells of which outwardly are 

 strongly thickened, the external layers colourless, and capable of 

 strongly swelling, the inner brown coloured and not swelling. In 

 sections laid in water the swelling layers, increasing in volume, 

 at length break through the cuticle, and project outwards like 

 papillae. It is these which make the wet seeds slippery. The 

 thick tissue underlying the epidermis appears in cross-section to- 

 be formed of polygonal cells, rounded at the angles, brown, and 

 pretty strongly thickened, and to it succeeds a layer of cells only 

 about one-third so thick, tangentially elongated, but less strongly 

 thickened. These impinge on a shining white, thick membrane. 

 This last arises from the strongly-thickened outer walls of the 

 outermost layer of the nucellus ; the entire remainder of the 

 spermoderm comes from the outer integument of the ovule. The 

 inner integument of the ovule is very early displaced. The nucel- 

 lar cells, the thickening layers of which we have ascribed to the 

 spermoderm, are mostly collapsed, as also are the rest of the cells 

 of the nucellus which are present. To this collapsed layer of 

 tissue succeeds a thin layer of endosperm, which ensheathes the 

 embryo. The endosperm-cells are densely filled with aleurone 

 grains. As successive surface sections show, the epidermis con- 

 sists of cells which are comparatively but little elongated, the 

 inner thickening layers of which are pitted. The tissue following 

 the epidermis, which in cross-section appears isodiametric, shows 

 now to be longitudinally elongated, and provided with oblique 



