NATURE AND STRUCTURE OF SEEDS 



57 



On the surface of seeds occur certain structures which suggest 

 the structural relation of the seed to the ovule. The micropyle, 

 the small opening through which the pollen tube entered the 

 ovule, persists as a tiny pit on the seed coat. Usually near the 

 micropyle there is a much larger scar, called the hilum, left where 

 the seed broke away from the funiculus, the stalk-like structure 

 which attached the ovule to the ovary and through which the 

 seed received food and water during its development. (Fig. 61.) 

 In case an ovule turns over on its elongated stalk and grows fast 

 to it, the stalk persists on the 

 seed coat as a distinct ridge, 

 called the raphe. (Fig. 62.) 

 In some seeds, like those of 



-h 



FIG. 61. Beans showing the 

 hylum at h and the micropyle 

 at m. 



A B c 



FIG. 62. A, seed of Pansy showing 

 raphe (r) . B, seed of Castor Bean show- 

 ing caruncle (c). C, seed of White 

 Water Lily showing the aril or loose 

 jacket around the seed. 



the Castor Bean, an enlargement known as the caruncle develops 

 near the micropyle. 



Structures such as hairs, plumes, hooks, and other appendages 

 which do not occur on ovules, are direct outgrowths of the seed 

 coat and function chiefly in dissemination. Similar appendages 

 occur often on one-seeded ovaries in which case one can tell only 

 by dissection whether the structure is a seed or one-seeded fruit. 



Many of the small one-seeded fruits are commonly called seeds. 

 In addition to a seed, they contain the ovary wall which persists 

 as an outer covering over the seed. The so-called seeds of Let- 

 tuce, Buckwheat, Ragweed, and the grains such as Corn, Wheat, 

 Barley, Rye, and Oats are familiar examples of one-seeded fruits 

 which are commonly called seeds. While they are not identical 

 with true seeds in structure, they are in function and therefore 

 may be appropriately discussed with seeds. In these one-seeded 

 fruits, the seed is protected by the hardened ovary wall, and 

 consequently, the seed coat is poorly developed, forming only a 



