1x6 THE STRUCTURE OF ECONOMIC PLANTS 



thirds of the length of the cell; but, near the inner tangential wall, 

 the radial walls are abruptly narrowed so that the lumen is wide. 

 The inner wall is porous and moderately thickened. (Fig. 107, C) 



The green seed coat consists of two layers. The outermost one 

 is comprised of tube cells which are distinct from the inner layer 

 owing to their elongated form and the extended intercellular 

 spaces. The inner layer consists of spongy parenchymatous cells 

 that are somewhat irregular in outline. In the mature fruit, the 

 perisperm and endosperm are each one cell layer in thickness and 

 adhere to each other; but, in a soaked fruit, they may be readily 

 separated from the seed coat and the embryo. The endosperm also 

 extends between the cotyledons and hypocotyl as a membrane 

 several layers in thickness. The cells of the endosperm contain 

 protein grains resembling the aleurone grains of cereals. 



The embryo, which is curved so that its axis is U-shaped, consists 

 of two plano-convex cotyledons enclosing a well-developed 

 epicotyl, a subcylindrical hypocotyl, and primary root. The 

 epidermal layers of the cotyledons are composed of small cells 

 containing aleurone grains. Beneath the abaxial epidermis are 

 several layers of isodiametric cells, and underlying the adaxial one 

 are two or more layers of palisade cells. (Fig. 107, C) The orien- 

 tation of the embryo is such that the tip of the primary root and 

 and the apices of the cotyledons are directed toward the micropylar 

 (chalazal) end of the seed, and the stylar end of the ovary. (Fig. 

 108, /4.) 



Development of the Seedling. — Upon germination, the 

 primary root emerges from the fruit coat at the stylar end, splitting 

 the coat into halves which remain united at the base, enclosing the 

 cotyledons until they are pulled above the ground by hypocotyle- 

 donary growth. (Fig. 109.) The primary tap root grows very 

 rapidly; and, under favorable conditions, may reach a length of 

 7.5 to 10. o cm. within a period of 48 hours after emergence. This 

 rapid rate of elongation is then retarded; and, as early as the third 

 or fourth day, there is a marked development of lateral roots which 

 arise just below the soil surface. At this time, the hypocotyl is 

 also elongating and becomes erect. The epicotyl is pubescent and 

 oval in transection, increasing slightly in diameter toward the 

 cotyledonary node. The expansion of the cotyledons is rapid after 

 they become free from the fruit coat; and, according to Lubbock 

 (2.0), they are approximately equal in width but differ in length. 



