THE GRAIN OF WHEAT. 41? 



The two cases studied above are both examples of exalbumin- 

 ous seeds, i.e., of seeds in which the endosperm formed in the 

 embryo-sac is completely, or almost completely absorbed into the 

 embryo during its early growth, and the reserved food stored up 

 mostly in the cells of the cotyledons. Examples of the other 

 primary, but less common, type of seed, known to systematists as 

 the albuminous seed, i.e., in which the endosperm formed in the 

 embryo-sac is not absorbed by the embryo until the period known 

 as germination, have been already in part studied by us in Chap. II., 

 especially in the case of Triticum as a Monocotyledon, and Ricinus 

 communis, the Castor Oil, as a Dicotyledon. It is advisable for 

 the student to amplify his knowledge of the structure of seeds 

 by a careful study of the ripe seed of this latter plant, first 

 removing the outer spermoderm, or testa, as it is too hard for 

 satisfactory section-cutting. Within this testa is a membranous 

 inner spermoderm the tegmen. The embryo is straight ; the 

 two large, delicate, veined cotyledons lie on either side of a narrow 

 central cavity, which nearly separates the seed into two halves ; 

 the radicle projects towards the curious, wart-like micropylar 

 aril which will have been noticed at one end of the testa. The 

 embryo is embedded in oily endosperm. The information given 

 in the above chapter will enable the student to fill in the details of 

 the structure of the seed. 



The Grain of Wheat. On account of its special interest, we 

 will, however, mata a full study of the " grain " of wheat, the 

 fruit of Triticum vulgare. The ovary of the flower of the wheat 

 plant is superior, or free, and contains a single ovule. This latter, 

 in its development, completely fills the cavity of the ovary, the 

 pericarp adhering to the spermoderm, as is shown in Fig. 12, 

 p. 30, and both being strained and flattened, so that the constituent 

 cells collapse. We may investigate either softened, or, what is 

 even better, just ripe grains. If we take softened grains, we must 

 take care that the grain is only just soft enough to make it suitable 

 to cut. The ripe grain of wheat shows in the middle line of its 

 inner side a deep furrow, corresponding with the ventral suture of 

 the ovary. At the base of the opposite side is the embryo, recog- 

 nisable as a slightly elliptic protuberance ending conically below. 

 The flattened top of the grain is crowned with hairs, between 

 which is the filiform residue of the style. The " grain " of wheat 

 is not a naked seed, but a one-seeded, dry, indehiscent fruit a 



caryopsis. 



27 



