SPERM ATOPHYTA 77 



the tube cell, piercing the stigma, grows down its stalk and pene- 

 trates an ovule, permitting the passage thither of the other two 

 cells (Fig. II, E'). By this time the nucleus of the ovule has 

 also divided several times, forming several cells ; to one of these 

 cells one of the two male cells is attracted, and there results a 

 fusion of the two cells into one ; this fusion is fertilization. 

 Immediately this new cell, nourished by the other cells, grows 

 rapidly into a minute embryo. This embryo consists of a 

 stem with seedling leaves (cotyledons) at one end, and a root 

 at the other. In this state, the seed, it ceases growing and may 

 remain dormant for years. As animals prepare food material 

 — the yolk — for the growing embryo, so also do plants ; in the 

 grasses and palms this nourishment lies in the seed outside the 

 cotyledons, in the bean and walnut within the cotyledons them- 

 selves. 



It is thus seen that the pollen grain and ovule always develop 

 into distinct plants of a few cells each. These few cells form the 

 sexual or gametophyte stage of the angiosperms, corresponding 

 to the entire liverwort and moss plant and to the prothallus of 

 ferns. The union of two cells, one from each of these plants, 

 produces the embryo which under favorable conditions will 

 develop into the adult plant, — the asexual, or sporophyte 

 stage; it, like the fern-plant or the little capsule of the moss, 

 produces spores called here the pollen and ovules. 



Both classes of the angiosperms made their first appearance, 

 so far as known, in the Upper Comanchean. Here appeared 

 such modern dicotyledonous genera as the poplar, willow, laurel 

 and fig, as well as primitive representatives of such monocoty- 

 ledonous families as the pondweed and the sedge. This early 

 flora is best known from the Upper Potomac formation (Patap- 

 sco) of Virginia and from strata of corresponding age in Portu- 

 gal. Later upon the Cretaceous lands flourished very lux- 

 uriantly many of our best known living plants, — the oak, 

 walnut, beech, birch, holly and ivy of the dicotyledons, the lily 

 and palm of the monocotyledons. The rapid rise of the angio- 



