n8 



PART I. — ORGANOGRAPHY. 



constitutes the aril, as in the mace of the Nutmeg, Fig. 346, the 

 aril of the Water-lily, and that of Celastrus scandens ; or, lastly, 

 it may be in the form of a cellular excrescence at the hilum or 

 along the raphe, called the caruncle, or crest, as in the seed of 

 Sanguinaria, Fig. 347. Not infrequently, also, the outer portion 

 of the testa undergoes change into mucilage, as in the seeds of 

 the Quince and Flax. 



The inner coat of the seed, called the tegmen or endopleura, is 

 sometimes wanting, sometimes it coalesces with the outer coat 

 and becomes indistinguishable from it, or it may unite with the 

 nucleus. When present, it is usually thin and membranous. 



Fig. 347. 



Fig. 348. 



Fig. 349. 



Fig. 347. — Carunculate seed of the Blood-root. Magnified. 



Fig. 348. — Longitudinal section of acorn, showing the exalbuminous seed. 



Fig. 349. — Albuminous seed of Hyoscyamus, shown in longitudinal section. Magnified. 



(2) The internal structure of the nucleus undergoes import- 

 ant changes. Some of these were mentioned under the subject 

 of fertilization. 



The nucleus of the seed, or that portion within the seed-coats, 

 may, as we have already seen, consist either entirely of the 

 embryo, in which case it is described as exalbuminous, as in the 

 Acorn, Fig. 348, or of the embryo, together with more or less 

 nourishing matter called, according to its origin, either endosperm 

 or perisperm, in which case the seed is termed albuminous, as in 

 Fig. 349, which represents a section of the seed of Hyoscyamus. 

 The term endosperm, as already explained, is applied to the 

 nutritious matter developed outside the embryo but within the 

 embryo-sac, while perisperm is applied to the nutritious matter 

 stored up outside of the embryo-sac. Both serve the same pur- 

 pose, and in the fully developed seed it is often difficult, if not 

 impossible, to distinguish them. 



