STRUCTURE OF THE SEED. 

 Fig. 104. 



211 



Fig. 104. Anatropal seed of the Orange, {Citrus aurantium,) opened to show the 

 cbalaza, c, which fonuB a brown spot at one end ; r, raphe, or internal funiculus rami- 

 fying in the rugose or wrinkled testa of the orange. 



On the outside of the episperm there is softietimes an addi-- 

 tional envelope formed, after the fertilization of the ovule, by 

 an expansion of the funiculus at the hilum. This funicular 

 expansion, which covers more or less of the surface of the 

 episperm, is termed an aril. The aril is very conspicuous in 

 the Spindle tree or Burning bush, (Euonymus,) where it forms 

 a beautiful scarlet envelope to the seed. The tough, fleshy 

 and lacerated body which invests the seed of the nutmeg, 

 known in commerce under the name of mace, is an aril. 



The nucleus, or kernel. This is all that part of the ripe 

 seed which is enveloped by the episperm. It is formed by the 

 development of the nucleus of the ovule, and like that organ is 

 attached to the episperm by its base, which forms the chalaza. 

 Grenerally, in the ripe seed this communication is destroyed. 

 The kernel in a fecundated seed always contains an embryo. 



In exalbuminous seeds, after fecundation, the embryo takes a 

 considerable development, absorbing into its cotyledons the 

 nutritive matter of the nucleus, so as ultimately to constitute 

 the entire kernel, as in the pea and bean. In albuminous 

 seeds, the embryo appears to be arrested in its growth whilst 

 yet in a minute and rudimentary condition, developing only so 



