268 MANUAL OF BOTANY 



a little lid at the period of germination : this has been termed 

 the emhryotegia. 



On removing the testa we observe the raphe, which fre- 

 quently ramifies over the inner coat, and where it terminates it 

 constitutes the chalaza {figs. 583, ch, and 577, ch). The structure 

 and general appearances of these different parts have been already 

 described. 



b. Tegmen, Endopleura, or Internal Coat (fig. 577, e). — The 

 inner menabrane or coat of the seed is essentially parenchy- 

 matous like the outer. In many cases, however, it seems to 

 be altogether wanting, which probably arises from its com- 

 plete adherence to the testa. 



"When clearly distinguishable the tegmen is generally of a 

 soft and delicate nature, although sometimes it is 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 kernel of the seed, which it accompanies in all 

 its foldings and windings ; and in some cases even dips down 

 into the latter, and thus divides it more or less completely 

 into a number of parts, as in the Nutmeg and Betel-nut {fig. 

 589, i^;. 



Arilhbs. — Besides the two integuments described above as 

 those that are usually found in all seeds, we occasionally find 

 on the surface of some seeds an additional integument, which 

 generally extends only partially over it {fig. 583, a, a), and 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 

 fertilisation has taken place. Two kinds of aril have been 

 described, 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 ovule already described, that 

 is to say, it makes its first appearance around the hilum in the 

 form of an annular process derived from the placenta or funi- 

 culus, and gradually proceeds 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 Nym.phaia {fig. 583, 

 A, a). The false arillus or arillode arises from the mioropyle, 

 and seems to be a development or expansion of the exostome, 

 which gradually extends itself more or less 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 ariUode in the seed of the 



