OEGAXS OF REPRODUCTION. 331 



arises from its complete incorporation or adherence to the testa. 

 Sometimes the embrj^o-sac in the ripe seed remains distinct from 

 the albumen of the nucleus {fig. 742), and remains in the form 

 of a bag or sac which envelopes the embryo, as in the Nym- 

 phseacege, Piperacese, and Zingiberacese, To this distinct mem- 

 brane the name of vitellus has been given. 



The endopleura is generally of a soft and delicate nature, al- 

 though sometimes of a fleshy character, either entirely or in part. 

 It is usually of a whitish colour, and more or less transparent. 

 This layer is closely applied to the nucleus of the seed, which it 

 accompanies in all its foldings and windings ; and in some cases 

 even dips down into the albumen of the nucleus, and thus divides 

 it more or less completely into a number of parts, as in the Nut- 

 meg and Betel-nut {fig. 7-i3, p). (See Albumen-, p. 335.) 



The testa may either accompany the endopleura in its wind- 

 ings ; or, as more frequently happens, especially when the 

 nucleus is curved, the endopleura only follows the nucleus, the 

 testa remaining in an almost even condition. 



Arilhis. — Besides the two integuments described above, as 

 being usually found in all seeds, we occasionally find on the sur- 

 face of others an additional integument, usually of a partial na- 

 ture {fig. 737, A, a), to which the name of arillus or aril has been 

 given. No trace of this structure is present in the ovule till after 

 the process of fertilization has taken place. Two kinds of arillus 

 have been described by St. Hilaire and Planchon, which have 

 been respectively called the true arillus, and the false arillus or 

 arillode. These have an entirely different origin ; thus, the true 

 arillus arises in a somewhat similar manner to the coats of the 

 o\'ule already described, that is to say, it makes its first ap- 

 pearance around the hilum in the form of an annular process 

 derived from the placenta or funiculus, and which gradually pro- 

 ceeds upwards, so as to produce a more or less complete additional 

 covering to the seed, on the outside of the testa. This arillus 

 is well seen in the Nymphtsa {fig. 737, a, a). 



T)a.Q false arillus or arillode, according to the investigations of 

 St. Hilaire, and the more recent elaborate ones of Dr. Planchon, 

 arises from the micropyle, and seems to be a development or ex- 

 pansion of the exostome, which gradually extends itself over the 

 testa to which it forms a covering, and after thus coating the 

 seed, it may be even bent back again so as to enclose the 

 micropyle. The gradual development of the arillode in the 

 Spindle-tree {Euonymus), is well shown in fig. 738. In the 

 Nutmeg, the arillode forms a scarlet covering to the testa, which 

 is commonly known under the name of ynace. According to Miers, 

 the arillode in the Euonymus is produced from the funiculus and 

 not from the exostome, in which case it would necessarily be an 

 arillus and not an arillode as commonly described. 



