328 



SEED OR MATURE OVULE. 



The Endopleuea is also cellular. It is often thin and trans- 

 parent, but it sometimes becomes thickened. It is applied more or 

 less closely to the embryo, and sometimes follows a sinuous course, 

 forming folds on its internal surface, and separating from the episperm. 



When the embryo-sac remains distinct from the nucleus in the 

 seeds, as in Nymphsea, Zingiber, Piper, etc., it forms a covering to 

 which the name of vitellus (vitellus, yolk of an egg) was given by 

 Gsertner. 



Aeillxjs. Sometimes there is an additional covering to th'e seed, 

 derived from an expansion of the funiculus or placenta after fertilisa- 

 tion, to which the name arillus has been given. This is seen in the 



Pig. 677. 



Passion-flower, where the covering commences at the base, and proceeds 

 towards the apex, leaving the foramen uncovered. In the Nutmeg 

 and Spindle-tree this additional coat is said to commence at the side 

 of the exostome, and to proceed from above downwards, constituting, 

 in the former case, the substance called mace ; and, in the latter, the 



Fig. 678. 



bright scarlet covering of the seeds (figs. 577, 578). In such instances 



Fig. 577. 1, 2, 3, 4, Various states of the arillus of the spindle-tree (Euonymus). The 

 figures show the mode in which it is developed from the edges of the foramen, a a a a, Aiil- 

 lode. ////, Foramen or Exostome. 



Fig. 578. Development of the same aiillus, a, around the ovule, o, exhibited in a different 

 position. 1, 2, 3, 4, are foxir successive stages of development. In fig. 4 the arillus has been 

 cut vertically to show its relation to the ovule, which it surrounds completely. 



