ia8 Plant Life and Evolution 



penetrates the tissue overlying the embryo-sac very 

 much as the fungus bores its way through the tissues 

 of its host. In many of the conifers there is an 

 interim of nearly a year between the time of pol- 

 lination and the penetration of the pollen-tube into 

 the archegonium, which is formed long after the 

 pollen first falls upon the ovule. 



Fertilization. With the discharge of the sperm 

 nucleus into the egg, and its fusion with the egg- 

 nucleus, fertilization is consummated, and the egg 

 develops into the embryo-sporophyte, which at once 

 begins to grow until the young organs stem, roots, 

 and leaves are well advanced. As is the case in 

 Selaginella, only a portion of the egg develops into 

 the embryo proper, a greater or less amount going 

 to form a peculiar organ known as the suspensor, 

 which pushes the developing embryo into the mass 

 of gametophytic tissue, the " endosperm," whose 

 cells are filled with starch, oil, and albuminous re- 

 serve food for the needs of the young sporophyte 

 when the seed germinates (Fig. 14, D). The wall 

 of the sporangium, together with the accessory en- 

 velopes, or integuments, which are found in nearly 

 all seed-plants, hardens and forms the characteristic 

 shell, or testa, upon the outside of the seed. It is 

 thus clear that the seed is not a new formation, 

 peculiar to the seed-plants, but is a more or less 

 perfectly changed megasporangium, within which 

 is contained the megaspore with its enclosed game- 

 tophyte, in which in turn is embedded the embryo- 



