NATURE AND STRUCTURE OF SEEDS 



57 



On the surface of seeds are present a number of structures, 

 which are in part vestigial ovular structures and in part develop- 

 ments accompanying the transformation of the ovule into a seed. 

 The micropyle, the small opening through which the pollen tube 

 commonly enters the ovule, persists on the seed coat as a smaU 

 pit resembhng a pin hole. Usually near the micropyle is a 

 conspicuous scar, called the hilum, left where the seed broke 

 away from the funiculus, the stalk-hke structure by which 

 ovules and seeds are attached to the ovary and through which 

 they receive food and water 

 during development. (Fig. 

 61). In some plants the 

 ovules curve so much during 



Fig. 61. — Beans showing the 

 hylum at h and the micropyle 

 at TO. 



A B ^' 



Fig. 62. — A, seed of Pansy showing 

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

 ing caruncle (c). C, seed of White 

 Water LUy showing the aril or loose 

 jacket around the seed. 



development that they are bent back against their stalks which 

 sometimes become attached to the seed coat, forming a ridge, 

 called raphe. (Fig. 62.) In some seeds, like those of the Castor 

 Bean, an enlargement, called caruncle, develops near the micro- 

 pyle. In preparation for ;'dissemination, often hairs, spines, or 

 other appendages grow out from the seed coat as the seed develops. 

 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. Many of the one-seeded 

 fruits resemble seeds so much that it is only by dissecting and 

 finding the seed within that they can be told from seeds. The 

 so-called seeds of Lettuce, 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, 



