114 THE STRUCTURE OF ECONOMIC PLANTS 



be present, the nucellar membrane, the aleurone layer, the starchy 

 endosperm, and the embryo. The pericarp has been described by 

 Winton (15), Randolph (15), and others. It has several layers, 

 the outer zone or epicarp consisting of elongated cells with thick 

 porous walls and a well-developed cuticle covering the outer sur- 

 face. The mesocarp or central zone is several layers thick and the 

 cells resemble those of the epicarp, except that the walls are some- 

 what thicker. Underlying this is the spongy parenchyma, con- 

 sisting of cells with radiating arms which are joined in such a way 

 as to form relatively large intercellular spaces. This zone is much 

 more pronounced at the pointed end of the grain. Adjacent to the 

 parenchyma is a layer of tube cells. These are the inner epidermal 

 cells of the pericarp which are drawn apart laterally as the grain 

 enlarges and give the appearance of a network over its inner face. 

 Within the pericarp, there may or may not be some remnants of the 

 integuments. The outer integument, which never completely 

 encloses the nucellus, is crushed or resorbed early in development; 

 but, in some cases, portions of the inner integument persist. This 

 integument is two-layered except near the micropyle, where there 

 are three to five layers. Weatherwax (2.1) found crushed remains 

 of this tissue in the mature grain and referred to it as a testa. Ran- 

 dolph (15) made an ontogenetic study of the grain in the variety 

 Pride of Michigan and "concluded that the maize caryopsis has no 

 true seed coat." 



The nucellus in the mature grain consists of non-cellular remains 

 of the disintegrated tissue and a suberized membrane lying between 

 the pericarp and the aleurone layer. The endosperm consists of 

 an aleurone layer and the starchy-parenchyma. The thick-walled, 

 cubical, aleurone cells lie directly within the nucellar membrane; 

 and the zone is usually one cell layer in thickness, although it may 

 be two-layered at some points owing to periclinal divisions. The 

 starchy-parenchyma consists of thin-walled storage cells. Those 

 immediately abutting the aleurone layer are smaller than the more 

 centrally located ones which constitute the horny portion of the 

 endosperm. They are higher in protein content than the starchy 

 endosperm and contain large numbers of small polygonal starch 

 grains. In the larger cells of the starchy endosperm, the grains are 

 for the most part rounded, and the larger ones have a distinct hilum. 

 The amount of horny and starchy endosperm varies greatly with 

 the variety of corn, as do the percentages of carbohydrates, pro- 



