368 



THE STRUCTURE OF ECONOMIC PLANTS 



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and lined with a peripheral layer of endosperm. The starch is still 

 restricted to the outer integument, but it is increased in amount 

 near the micropyle and at the chalaza. (Fig. i86, C) At about 

 five days, the embryo occupies a position in the curved portion of 

 the embryo sac, and the endosperm constitutes a considerable pro- 

 portion of the seed, since the embryo sac is enlarged and the em- 

 bryo is relatively small. (Fig. i86, D.) The endosperm nuclei are 

 peripherally located, the central cavity of the embryo sac contains 

 a watery cytoplasm; and, although starch is present in increased 



amounts, it is still restricted 

 to the outer integument. 



About six days after pollina- 

 tion, the embryo differentiates 

 cotyledons which enlarge as a 

 result of an increase in the 

 number and size of cells, divi- 

 sions taking place somewhat 

 more rapidly near the pe- 

 riphery. Later, this meriste- 

 matic condition is restricted 

 to the tip and outer surface of 

 each cotyledon, and the cells 

 , at the center and toward the 



Fig. 187. Ovule thirty-six hours after pol- j • 1 r l J 



lination, showing development of embryo sac adaxial SUtiace enlarge and 



and peripheral position of endosperm. Dark cease dividing. It is at this 



areas indicate regions of starch deposition. , , , . . 



(Photomicrograph by Pultz.) Stage that starch deposition 



is initiated in the non-meriste- 

 matic regions of the cotyledons; and when the seeds are eight 

 or nine days old, starch appears in the middle region of the 

 cotyledons and at the cotyledonary plate. The peripheral cells 

 on the abaxial surface and tips of the cotyledons continue to divide 

 and contain little or no starch, as is also the case in the hypocotyl 

 and epicotyl. 



The developing embryo occupies a small part of the embryo sac, 

 the remainder being filled with a more or less watery, sweet-tasting 

 endosperm. Starch is not present in the endosperm at this or at 

 any other time during its development. The cotyledons continue 

 to grow actively by means of a peripheral band of meristematic 

 cells, and this is followed by a progressive deposition of starch 

 which explains the greater amounts in the middle of the cotyledons 



