28 3 



as in the Cotton, Willow, or Poplar. These are effective in the transfer of the 



seed b} r the wind. Succulent bodies may sometimes be formed by local 



hypertrophy ; such as the massive enlargement of the micropylar region to 



form the "caruncle" of the Spurges. A growth may proceed from the base of 



the ovule, appearing as an extraintegu- 



ment, or arillus (Fig. 225). The Mace 



sold by grocers is an example. It 



appears after fertilisation as a partial 



covering, highly coloured and strongly 



flavoured, round the true seed, which 



is sold as the Nutmeg. The bright 



orange sheath round the ripe seeds of 



the Spindle Tree are of the same 



nature. In both cases the aril is 



r. 



exposed as the fruit ripens, and its 

 presence is believed to promote distri- 

 bution of the seeds by Birds. But 

 such developments are infrequent. 



Each seed contains normally a 

 single germ, together with a store 

 of nutriment either in the germ 

 itself or in the accessory tissues. 

 (Fig. 226.) As it ripens it dries 

 out. In this state, after separation 

 from the parent it may undergo 

 a period of rest.. But sooner or 

 later its function is to establish a 

 new individual. In order that this 

 may be most effectively done it is 

 important that each seed should 

 have the chance of independent 

 development, a condition which is 

 secured by dispersal of the seeds. 



Each seed thus represents a single matured megasporangium, or 

 ovule. It is a composite body comprising parts derived frop three 

 generations. The seed-coat, and the perisperm when present, consist 

 of tissues of the parent diploid plant. The endosperm is usually 

 held as representing, in a specialised and altered state, the haploid 

 female prothallus, or gametophyte generation. The germ, which 

 results directly from fertilisation, represents the new diploid genera- 

 tion. That these are all closely related in the seed of Flowering 

 Plants is a late and derivative state, which cannot be properly under- 



ktood till certain lower types have been described. 



FIG. 226. 



Seeds in median section : the upper (Capsella) 

 exalbuminous, the lower (Datura) albuminous. 

 / = funiculus. w=micropyle. <= testa. e= endo- 

 sperm. c= cotyledons. pl= plumule, r- radicle. 

 (Enlarged.) (Dr. J. M. Thompson.) 



