THE HILUM 129 



the subject of the pericarp. More frequently, however, it is secured 

 entirely through the coats of the seed. 



The Seed-coats. — These may correspond to the coats of the ovule, 

 though usually the primine is found to have disappeared. When it 

 persists it takes the name of Tegmen, or Endopleura, the secundine 

 becoming the Testa, or Exopleura. Rarely the secundine also disappears 

 and the seed is Naked. The seed will also be naked when produced from 

 a naked ovule. 



The Micropyle. — When one or both of the coats persists, the point 

 where the foramen, now closed, existed becomes the Micropyle. 



The Aril. — Frequently the development of a new coat external to 

 the others is induced by fertilization, and this is known by the general 

 name of Aril. If it develop from the chalaza or a lower point, it is called 

 an Arillus, or True Aril; if from the micropyle, an Arillode, or False 

 Aril. 



Dissemination. — The provisions of the seed for securing dissemination 

 are in most respects comparable with those affecting the pericarp. 

 Wind-distribution is preeminent, that by fixation to passing bodies is 

 frequent, and that by means of an edible coat is rare. 



Fixation.— The fixation of seeds disseminated without the pericarp is 

 favored by their small size, enabling them readily to enter crevices and 

 cavities, and by peculiarities of surface which favor the same process. 



The large number and importance of medicinal seeds lend great 

 importance to their study by the pharmacognosist, and this is especially 

 true of the histology of all their parts. Inasmuch, however, as the 

 subject of histology has been referred to a separate portion of the work, 

 we shall here consider only such characters as can be distinguished 

 by means of an ordinary lens. 



The Hilum. — The hilum is in most cases readily perceptible, but is 

 occasionally found only by minute examination. It is to be studied as 

 to its position, size, form, surface, and color. Its position is sometimes 

 fixed with reference to the form of the seed, as at the larger or smaller 

 end, upon the broad side or on the edge, as well as with reference to the 

 micropyle, adjoining it, at the opposite end or at some intermediate 

 point. It is the last-mentioned character which determines the class 

 of seed as to its tropism (see Ovule). In size the hilum may be a slight 

 point, or it may cover a considerable portion of the surface. Its form is 

 frequently characteristic, as heart-shaped (Fig. 370) or linear and 

 channelled, as in Fig. 371. Its color frequently differs markedly from 

 that of the remainder of the seed. 

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