THE MEGASPORANGIUM AND SEED 



231 



nuclei, which by the fusion of two become seven. Of these, one 

 nucleus surrounds itself with a protoplasmic body to become the 

 egg, while two others are related to the so-called synergidae. Of 

 the remaining four nuclear structures, three belong to the antipo- 

 dals, a group of cells present in the base of the embryo sac or 

 prothallus, while the fourth, the product of the fusion of two nuclei 

 as mentioned above, becomes the 

 so-called endosperm nucleus, which 

 later develops the endosperm or 

 food substance of the ripened seed. 

 In certain of the chalazogamous 

 angiosperms (Fig. 171), notably 

 Casuarina and the hazel (Corylus), 

 tracheids are found present in the 

 nucellus. The most natural inter- 

 pretation of this condition is in 

 connection with the tracheary 

 apparatus in certain extinct seeds 

 described in a foregoing paragraph. 

 If the presence of tracheary tissues 

 in the substance of the megaspo- 

 rangium or nucellus in certain 

 angiosperms is to be interpreted as 

 the persistence of an ancestral char- 

 "acter, it would indicate a relatively 

 primitive position for the chalazog- 

 amous forms in which it occurs. 



The structure of the mature seed in the angiosperms naturally 

 claims a greater interest in a work devoted to anatomy. We may 

 first take the cases of dicotyledons and monocotyledons. Fig. 172 

 illustrates the organization of the seminal organ of the Indian 

 corn (Zea). To one side lies the embryo, which is provided with a 

 single seminal leaf or cotyledon. This is very large in size and has 

 its inner surface applied broadly to the food substance of the seed 

 or endosperm. The embryo proper lies to the outside of the 

 cotyledon and is characterized by the presence of the primary shoot, 

 or plumule, and the primary root, or radicle. These are inclosed 



FIG. 172. Seed of Zea mats 



