MODES OF REPRODUCTION IN PLANTS. 



169 



termed, becomes inclosed with one or more coats of varying thickness, 

 and, when the whole structure has reached maturity and is ready to 

 separate from the parent-plant, we have the familiar body known as a 

 seed. The seed is an independent plant-structure designed to develop 

 into a mature herb, shrub, or tree, when the conditions are favorable 

 for its germination and growth. 



The lowest class of flowering plants is the Gymnosperms — which 

 includes the cycads and cone-bearing plants. The ovules are naked, 

 and the embryo develops considerable endosperm. This corresponds 

 to the prothallus of higher cryptogams, as in it the corpuscula, which 

 correspond with archegonia, are formed. The pollen-grains are several- 

 celled, thus suggesting a prothallus, especially as only a part of the 

 grain takes part in the formation of the tube and the male fertilizing 

 fluid. The gymnosperms evidently occupy an intermediate place be- 

 tween the higher cryptogams and the angiosperms or flowering plants 

 with their ovules inclosed in an ovary. This last class contains the 

 great mass of plants with evident floral organs, and is divided into the 

 exogens, like the oak, apple, and rose, and the endogens, illustrated by 

 the grasses and cereal grains. 



The points that interest us most in the present consideration are, 

 that, unlike the gymnosperms, the ovules are inclosed in an ovary ; the 

 endosperm forms in the embryo sac after fertilization, and the pollen- 

 grain sends out its tube without previous cell-division. The pollen-grains 

 often have no rudimentary jsro- 

 thallus. The first result of fer- 

 tilization is the formation of a 

 cellulose wall around the germ- 

 cell. This cell soon divides, 

 forming the suspensor at the 

 lower end, from which the em- 

 bryo plantlet is developed. The 

 endosperm - cells form at the 

 same time at the opposite end 

 of the embryo sac. Fig, 17, A, 

 shows a longitudinal section of 

 a young ovule shortly after fer- 

 tilization. The embryo sac is 

 at e, with a small embryo at 

 the left end, and free endo- 

 sperm-cells formed at the other. The embryo is shown more magnified 

 at B, and at C is seen the same more advanced. The endosperm is rich 

 in food-materials for the growing embryo, and may be entirely absorbed 

 and the space occupied by the latter. 



It now remains for us to determine the extent of the sexual genera- 

 tion in the flowering plants. Among gymnosperms it is not difiicult to 

 see that it consists of the pollen-grain and the embryo sac with its endo- 



FiG. 17. 



