ANGIOSPERMS. 



395 



In Monocotyledons and many Dicotyledons the embryo within the endosperm 

 remains small, and is either enveloped by it or is placed on one side of it, as in the 

 Gramineae ; the cells of the endosperm leave no intercellular spaces and are filled till 

 the seed is ripe with protoplasmic substance and fatty oil or starch or both, and in 

 this case remain thin-walled ; the endosperm then appears as the mealy (rich in 

 starch) or oily kernel of the ripe-seed, with the embryo in the middle of it or by its 

 side ; it sometimes becomes horny from a considerable thickening of its cell-walls 

 which have the power of swelling, as in the Date and other Palms, in the Umbelliferae, 

 Coffea, etc. ; if this thickening is excessive, the endosperm may fill the seed-coat as a 

 stony substance, as in Phytelephas, 'vegetable ivory'; in these cases the thickening- 

 masses of the cells are dissolved during germination and serve with the protoplasmic 

 and fatty cell-contents to feed the young plant. The endosperm, when fully formed 

 and copiously developed, has usually the shape of the entire ripe seed and is uniformly 

 covered by its coat ; its external form therefore is usually simple and often rounded ; 

 but considerable deviations from this condition occur not unfrequently and especially 

 among the Dicotyledons ; this is the case, for example, in the well-known coffee- 

 bean (Coffea), which with the exception of the minute embryo concealed in it is entirely 

 composed of the horny endosperm ; but this, as a transverse section shows, is a plate 

 with its edges folded inwards. The marbled 

 appearance of the ruminate endosperm which 

 forms the kernel of the nutmeg, the seed of 

 Myristica fragrans, and of the Areca-nut, the 

 seed of the areca palm, is due to an inner dark 

 layer of the seed-coat which grows inwards in 

 the form of radiating lamellae into narrow folds 

 in the clear endosperm. The ripe endosperm 

 is either a solid body of tissue, or it has a 



J FIG. 325. Polygonum Fagopyrum. To the left a 



cavity, which in Strychnos nux-vomica, for transv e rs =. to the "^t a longitudinal sect through 



* * -^ a mature ovary ; dried stigmas, s testa, e endosperm. 



instance, is a broad flat narrow fissure; in these ^ cotyledons folded ma sinuous manner, as is shown in 



the transverse section, TV root. 



internal cases the endosperm as it grows from 



the outside of the embryo-sac inwards leaves a central space unfilled; this space 

 in the coco-nut, as has been already mentioned, is large and contains a fluid, while 

 the endosperm itself is a hollow thick-walled sac enclosing a roundish or fissure- 

 like cavity. 



In very many families of the Dicotyledons the first leaves of the embryo, the 

 cotyledons, grow before the seed is ripe into bodies of such a size that they displace 

 the endosperm which is already formed, and at length fill the whole space enclosed by 

 the embryo-sac and the seed-coat, while the axial part of the embryo and the bud 

 which lies between the bases of the cotyledons remain comparatively small; such 

 cotyledons are thick and fleshy or leaf-like and then usually folded, and contain the 

 reserve of protoplasmic substance and starch or fatty matter which in other cases is 

 stored up in the endosperm to be used during the unfolding of the young plant. 

 The cotyledons appear to obtain this copious supply of food-material from the 

 endosperm, and therefore the difference between the seed which in its mature state 

 has no endosperm and one which contains endosperm is simply, that in the former 

 the reserve of food in the endosperm has passed into the embryo before germination, 



