THE EMBRYO OF ANGIOSPERMS. 421 



these two cells stretches, and is repeatedly segmented; thus a string of cells is 

 formed, known as the suspensor, bearing at its lower extremity the embryo -cell, 

 which gives rise to the greater portion of the young plant. The suspensor, by its 

 elongation, brings the embryo -cell well down into the cavity of the embryo-sac, 

 where it is embedded in the substance of the endosperm which has meanwhile 

 developed. The nutrition of the young plant is thus assured during its early 

 stages. In many Parasites and in Orchids the full-grown embryo shows but little 

 differentiation, and is little more than a mass of cells exhibiting no distinction of 

 stem and leaf -structures; but in the great majority of Angiosperms it soon shows a 

 differentiation into parts — into a little root at one end and a stem at the other, with 

 the rudiments of leaves (c/. vol. i. p. 599, figs. 141 ^ and 141 ^). These leaves, the 

 cotyledons, are a conspicuous feature of the embryo, and in several plants they are 

 coloured green by a precocious development of chlorophyll in their tissues (e.g. 

 Styphnolobium japonicum). In a large number of plants, as, for instance, the 

 Apple and Oak, Bean and Pea, garden Nasturtium (Tropceolum) and Water 

 Chestnut (Trapa, cf. vol. i. p. 607, figs. 144 ^- ^' ^' *■ ^' ^), the cotyledons become much 

 enlarged and succulent, and take up large quantities of food - material, which 

 remains stored up in them as reserve-material for the further development of the 

 young plant at germination. When this happens the cotyledons usually come to fill 

 the whole cavity of the seed right up to the integument {testa). In the majority 

 of plants, however, the cotyledons remain small and thin, and do not take up the 

 whole reserve of food-material which envelops the embryo. Under these circum- 

 stances the embryo is provided with a special reserve-tissue, destined for its con- 

 sumption when it is separated from the mother-plant. This tissue is analogous to 

 the yolk of a bird's egg, and consists of a tissue of cells filled with fat, starch- and 

 proteid-granules. This food-tissue is variously known as endosperm, albumen, &c., 

 but the terminology associated with it does not reflect great credit upon the 

 Botanists who are responsible for the introduction of the various terms. 



The starting-point for this reserve-tissue consists of the nucleus which arises 

 in the embryo-sac from the fusion of the so-called polar nuclei (represented in 

 figs. 316 ^' ^' ^, and described on p. 416). Around these nuclei a certain amount 

 of protoplasm collects, and after their fusion into the so-called definitive nucleus 

 (fig. 316 ^), a very active cell-division sets in, which results in the formation of a 

 parenchymatous tissue which occupies the embryo-sac and becomes filled with food- 

 materials (fat, starch, and proteids). This tissue is the already-mentioned reserve- 

 tissue of the seed or endosperm. Most frequently, as we have stated, the embryo 

 enters on a resting period embedded in or adjacent to this reserve, and absorbs it 

 at germination. In other cases, as in the Bean, Oak, &c., as mentioned, the 

 cotyledons of the embryo forthwith take up all this food, so that when the resting- 

 .stage comes on, the greatly swollen embryo fills the whole cavity of the seed. This 

 former class of seed is spoken of as albu/minous, the latter as ex-albv/minous. There 

 is just this difference between albuminous and ex-albuminous seeds: in the former 

 the embryo only takes up the food-material at germination, in the latter, relatively 



