348 



GENERAL BOTANY 



perianth is usually composed of modified sporophylls, but in 

 some flowers the leaves immediately below the sporophylls have 

 been transformed into sepals and petals. 



Pollination devices are more highly developed in the angio- 

 sperms than in any of the gymnosperms, and the pollen tube 

 traverses the tissues of the style, stigma, and ovary cavity before 

 coming in contact with the micropyle. The development of a 

 closed pistil with a receptive stigmatic surface has therefore greatly 

 modified both pollination and the growth of the pollen tube. 



The gametophytes are greatly reduced in size and in cellular 

 differentiation. The male gametophyte is represented by the 

 single generative cell, which gives rise to the two male cells 



Selaginella 



Spruce 



Mandrake 



Nuclei 

 r *~ Cytoplasm 



'I Gametophyte,^ 



FIG. 205. Diagram illustrating the homologous, or corresponding, structures of 

 microspores and male gametophytes in Selaginella, spruce, and mandrake 



a-c, microspores ; d-f, germinated microspores and gametophytes 



within the germinated pollen grain, or microspore. The female 

 gametophyte is reduced to the egg apparatus, polar nuclei, and 

 antipodals in the embryo sac. The embryo develops from the 

 fertilized egg cell within the embryo sac and passes into a rest- 

 ing stage within the seed. The full development of the sporo- 

 phyte begins with seed germination, when the embryo resumes 

 its growth, fed by the endosperm stored up around the embryo 

 or within its cotyledons. 



SUMMARY AND COMPARISONS 



A brief summary and comparison of the heterosporous plants 

 from Selaginella to the angiosperm will suffice to indicate the strik- 

 ing advances made by the highest spore-bearing and seed plants in 



