ANGIOSPERMS. 587 



the endosperm may fill up the testa as a hard mass, forming, for instance, the 

 * vegetable ivory ' in the Phytelephas. In these cases the thickened walls of the 

 endosperm-cells, which are absorbed during germination together with their proto- 

 plasmic and fatty contents, serve for the first nourishment of the embryo. The 

 ripe endosperm, when copiously developed, has usually the form of the entire 

 ripe seed, being uniformly covered by its testa; its external form is therefore 

 generally simple, often round ; although considerable deviations from this frequently 

 occur, especially among Dicotyledons. Thus, for instance, the substance known as 

 the ' coffee-berry ' consists, with the exception of the minute embryo which is con- 

 cealed in it, entirely of the horny endosperm; but this, as a transverse section shows, 

 is a plate folded inwards at its margins. The marbled (ruminated) endosperm which 

 forms the nutmeg (the seed of MyrisHca fragrans) and the areca-nut (the seed 

 of the Areca-palm) owes its appearance to the circumstance that an inner dark 

 layer of the testa grows in the form of radiating lamellae between narrow fold- 

 like protuberances of the light-coloured endosperm. The ripe endosperm is either 

 a perfectly solid mass of tissue, or it possesses an inner cavity, as in Sirychnos 

 Nux-vomica, where, like the seed itself, it is broad and flat. This is clearly the 

 result of the endosperm which grows inwards from the periphery of the embryo-sac, 

 leaving a free central space, which, as has already been mentioned, is very large 

 and filled with fluid in the case of the cocoa-nut. In these cases the endosperm 

 is therefore a hollow thick-walled sac, enclosing a roundish or flattened cavity. 



In a large number of families of Dicotyledons, the first leaves of the embryo, 

 the Cotyledons, grow, before the seeds are ripe, to so considerable a size that they 

 displace the endosperm which was previously present, and finally fill up the whole 

 space enclosed by the embryo-sac and the testa ; while the axial part of the embryo, 

 and the bud (plumule) that lies between the bases of the cotyledons, attain even 

 in these cases only inconsiderable dimensions. In these thick fleshy or foliaceous 

 cotyledons (which are then usually folded) the reserve of food-material accumulates, 

 consisting of protoplasmic substance, of starch and fatty matter, which is in other 

 cases stored up in the endosperm, and is made use of during the development 

 of the seedling. This storing of the cotyledons with so large a quantity of 

 food-materials appears to take place by its transference from the endosperm ; and 

 hence the diff"erence between those seeds which in the ripe condition contain no 

 endosperm [* exalbuminous '] and those which do contain it [' albuminous '] con- 

 sists essentially only in the fact that the food-material of the endosperm has passed 

 over in the former case before germination into the embryo ; while in the latter case 

 this only takes place during the process of germination. The presence or absence 

 of the endosperm in ripe seeds is more or less constant within large groups of 

 forms, and is therefore of value in classification. Of the better-known families, for 

 example, the Compositae, Cucurbitaceae, Papilionaceae, Cupuliferse (the Oak and 

 Beech), &c. are destitute of endosperm. Sometimes also the embryo increases in 

 size to such an extent that the endosperm appears as a thin skin surrounding it. 



We must now recur to the oospore in order to follow the formation of the 

 Embryo. The cell-wall of the oospore coalesces superiorly with the wall of the 

 embryo-sac at its apical swelHng, its free end being turned towards the base of 

 the ovule; it then lengthens, and undergoes one or more transverse divisions. 



