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TEXTBOOK OF BOTANY 



thing that distinguishes the seed plants. The embryo 

 now lies within the tissues of its parent, the female plant ; 

 the latter lies within the macrospore sac, which in turn is 

 surrounded by the integument. All these parts together 

 make up the seed. Thus the seed is a complex structure, 

 whose different parts represent three generations. The in- 

 tegument and the macrospore sac belong to the same 

 generation as the tree, that is, to the old asexual generation. 

 The female plant belongs to the sexual generation; and the 

 embryo is the new asexual generation. 



Changes take place in all these parts as the seed ripens. 

 The integument develops into two seed coats : an outer one 



FIG. 76. A, a mature macrospore leaf bearing seeds, seen from the lower 

 side. B, the same, seen from above. C, a single seed with its wing. 



which is hard and resistant, and an inner one which is very 

 thin and papery. The tissues of the ovule (inside the integ- 

 ument) have been largely destroyed by the growth of the 

 female plant ; but something of them remains in the mature 

 seed in the form of a cap over the micropylar end of the 

 female plant. The female plant in turn has been partly used 

 as food for the embryo ; but much of its tissue remains, sur- 

 rounding the embryo except at the root end, its cells filled 

 with starch and other foods which the embryo will use later. 

 This food-containing tissue of the female plant is called en- 

 dosperm in the ripe seed. A seed usually contains but one 



