138 



MORPHOLOGY 



distinguished as perisperm. Seeds with both endo- 

 sperm and perisperm are comparatively few. Usu- 

 ally the endosperm grows at the expense of the 

 . ji nucellus and ulti- 



E-3rv'' mately obliterates 

 it. Such seeds are 

 said to be albumin- 

 ous, or with endo- 

 sperm. This iscom- 

 Fig. i22.-sections of Seeds mon among Mono- 



A with endosperm E. B with endosperm E and COtylCdOnS. In tlie 

 perisperm P. c Exalbuminous seed. ., Testa; majority of DicO- 

 e, embryo ; c, cotyledons ; w, radicle, J J 



tyledons, however, 

 the embryo develops at the expense of the endosperm, 

 so that the whole of the endosperm formed at the 

 outset becomes ultimately obliterated, and the seed 

 becomes exalbuminous, or without endosperm. The 

 integuments of the ovule also 

 share in these changes. The 

 two coats become fused to- 

 gether, and change more or 

 less in texture and colour, form- 

 ing what is known as the testa 

 (fig. 122, s). Further, in some 

 seeds the cells of the outer layer 

 of the testa grow to form hairs, 

 either all over the testa or on 

 a particular part of it. For 

 instance, in Cotton and Silk- 

 cotton tree the seeds become 

 covered all over with hairs which are known as cotton 

 fibres; in akanda (Calotropis giganted) (fig. 123), 

 karabi {Nerium odorum), in fact in most Asclepi- 

 adacecB and Apocynacece, the seeds are crowned with 

 a tassel or coma of hairs. In some seeds a new coat 



Fig. 123. — Ak^nis. {Calotropis 

 giganiea) 



