A NEW GROUP OF FLOWERING PLANTS. 



137 



mature and ready for fertilisation, the tube begins to grow again 

 through the nucellus, reaches the apex of the embryo-sac, and thrusts 

 between the neck-cells of the archegonia protuberances which 

 finally reach the oosphere. A number of small nuclei have been 

 formed in the extremity of the pollen-tube from the original antheridial 

 nucleus; these disappear in fertilisation, and their substance in some 

 way or other passes into the substance of the oosphere, which thus 

 becomes the oospore and is invested with a cell-wall. From the oospore 

 is produced the embryo. Meanwhile, the integuments of the 

 ovule, together with what remains of the nucellus, have become 

 thickened and hard or leathery, forming the coats of the seed, 

 which is now ready to leave the parent, carrying a miniature of it in 

 the enclosed embryo. The life history of a typical Gymnosperm, say 

 the Scotch Fir {Plniis sylvestris), will be as follows : — 

 / Scotch Fir Tree 



Asexual generation 



Sporangium 

 Spore 



stamen 



I 

 pollen-sac 



pollen-grain 



Sexual generation . 



prothallium sterile cells 



pollen-tube 



carpel 



ovillej?"^^""^ , 

 I (^ integument 



embryo-sac 



I 



I 

 endosperm 



I 

 archegonium 



I 

 oosphere 



oospore 



I 

 Scotch Fir Tree. 



The great difference, from this point of view, between the Gymno- 

 sperms and Vascular Cryptogams, is the complete merging of the 

 two generations in one individual, and the entire dependence of 

 the sexual generation for nourishment and support on the previous 

 asexual one. This results in the production of the seed in which 

 the second asexual generation is finally dispatched to start a separate 

 existence. 



Coming now to the highest group, the Angiosperms, the plants 

 with an ovary, we shall find important differences. 



As regards the macrosporangium, the nucellus is protected by 

 one or two integuments. In the simplest case a sub-epidermal 

 cell, which is the archesporium, also becomes the macrospore 

 (embryo-sac). This occurs in Lilium hidbifeyum, but is rare. We 

 usually find the type established by Strasburger. One sub- 

 epidermal cell, the archesporium, divides transversely. The 

 upper daughter-cell grows and divides, giving rise to parietal 

 and tapetal layers, while the lower, the mother-cell of the macro- 

 spore, divides by one or several shiny, transverse walls, and one of 

 the resulting series of cells, almost always the lowest, becomes the 



