VEGE TA TIVE REP ROD UCTION. 



235 



pollen, and the megaspores are called embryo -sacs .* The mi- 

 crosporangia and megasporangia, also, are always different in 

 form and structure, and the leaves upon which they are usually 

 borne are also of two distinct forms. In no case do sporo- 

 phylls perform nutritive work ; they are always specialized. 

 Those leaves which bear microsporangia are called stamens, 

 and the leaves which produce the megasporangia are called 



Fig. 242. — A , a microsporangium of Salvinia seen from the outside. It contains 64 

 microspores. B, four spores from A , surrounded by hardened frothy mucilage. C, 

 median longitudinal section of a megasporangium, showing structure of wall at matu- 

 rity, and the single spherical megaspore, with its proper wall (black line) and a thick 

 frothy epispore vH 308). A and C magnified 55 diam. B, magnified 250 diam.— After 

 Strasburger. 



carpels* (figs. 245, 250, 251). In spite of these special 

 names, it must be carefully borne in mind that the sporangia 

 and sporophylls of the seed plants are not different from those 

 of the fernworts or mossworts in any essential particular. 



329. The sporophylls of the seed plants are usually aggre- 

 gated by the failure of the internodes of the axis to lengthen 

 as much as between the foliage leaves. Very often, also, the 



* These special names were given because the seed plants were first 

 studied, and it was long before the real nature of the parts and their rela- 

 tion to similar ones in the lower plants were known. The terms are still 

 in use, and are likely to continue to be used for convenience. 



