ACCESSORY STRUCTURES OF THE SEED. 



425 



show very various characters, just as the testa itself may do. In the Willow (Salix, 

 fig. 319) and Poplar (Populus, figs. 318" and 318*) it consists of long, delicate silky 

 hairs; in many Passifloraceae, Sapindaceae, and Celastrinese— amongst others, in the 

 well-known Spindle-tree (Euonymus), it forms a fleshy, succulent investment, often 

 coloured bright red or orange, whilst in the Myristicacese it forms a curious, laciniated 

 sheath. In the Nutmeg {Myristica, moschata) the seed proper constitutes the 

 nutmeg of commerce, whilst the mace is the aril which grows around this seed. 

 When these accessory structures of the seed are only developed locally as little 

 ridges or bumps at the base of the seed, or on the funicle, they are known as 

 caruncles. A well-marked, fleshy cock's-comb-like caruncle is formed on the seeds 

 of the Celandine (Chelidonimn majus). When the swelling is limited to the point 

 of attachment of the seed to its funicle, one speaks of a hilwr caruncle, as is seen in 

 the Pansy (c/. figs. 320 ^ 

 and 320 2). The spot 

 where the seed is at- 

 tached to its stalk is 

 known as the hilum, and 

 is readily seen, even 

 when distinguished by 

 no special swelling or 

 caruncle, on a detached 

 seed. It is usually a 

 well-defined area, col- 

 oured differently from 

 the rest of the testa, 

 sometimes projecting, 

 sometimes slightly exca- 

 vated, and not infre- 

 quently having the form of a well-marked groove (see fig. 320^). At the place 

 where separation has occurred there is a kind of scar, the hilar scar (sometimes 

 called the omphalodium). The spot occupied by the micropyle is often recogniz- 

 able on the ripe seed, and may be termed the micropylar scar. It usually appears 

 as a little hole or pin-point depression bordered by peculiar tissue. In curved (cam- 

 pylotropous) seeds, the micropylar and hilar scars are seen near together, but not so 

 close as to be inseparable (figs. 320* and 320^; in 320^ the point indicates the posi- 

 tion of the micropylar, the excavation that of the hilar scar). In the Castor-oil plant 

 {Ricinus communis, figs. 320^ and 320*), the lips of the micropyle undergo con- 

 siderable growth, forming a little cushion or micropylar caruncle, very conspicuous 

 at the top of the seed. 



Corresponding to the externally visible hilar and micropylar scars is a curious 

 internal structure of this portion of the seed-coat. This will be more intelligible 

 when it is explained that in many cases the water needed by the resting embryo 

 for its further development can only be absorbed at these spots. The tissue here, 



Fig. 320.— SeedB with carunclea and hilar scars. 



1 Seed of Viola tricolor, a The same in longitudinal section. » Seed of Ricinus com- 

 munis. >4 The same in longitudinal section, ^ Seed of Physostigma venenosum. 

 ^ Seed of Aimmirta Coeculus. ^ The same in longitudinal section. (After Baillon.) 



