THE EMBRYO AND THE SEED 



283 



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

 seed b)' 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 extra integu- 

 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 

 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 from 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 speciahsed and altered state, tlie 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- 

 stood till certain lower types have been described. 



Fig. 226. 

 Seeds in median section : the uppci i(.\ipsciut) 

 exalbuminous, the lower {Datura) albuiuinoiis. 

 / = funiculus, w = micropylc. /-testa, f -endo- 

 sperm, c = cotvledons. />/= plumule, r radicle. 

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



