196 



OUTLINES OF PLANT LIFE. 



Fig. 151. — Section through three spore 

 case clusters of an aquatic femwort 

 (Salvinia ntzians). Each is cov- 

 ered by a double indusium. z, /, 

 two clusters consisting of small spore 

 cases, each containing 64 micro- 

 spores; a, a cluster consisting of 

 large spore cases, each containmg 

 one megaspore. Magnified 10 diam. 

 — After Sachs. 



between the spore cases containmg small spores and those containmg large 



spores (fig. 151). 



In the seed plants this difference in the spores is always found. The 



microspores are called pollen grains 

 and the megaspores after germination 

 are called embryo-sacs.* The spore 

 cases 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 

 spore leaves perform nutritive work; 

 they are always specialized. Those 

 leaves which bear pollen grains are 

 called stamens, and the leaves which 

 produce the megaspores are called 

 carpels* (figs. 156, 157). In spite of 

 these special names, it must be care- 

 fully borne in mind that the spore cases 

 and spore leaves of the seed plants are 

 not different from those of the fernworts 

 or mossworts in anyessential particular. 



277. The spore leaves of the seed plants are usually 

 clustered by the failure of the internodes of the axis to 

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

 also, the leaves adjacent are modified in form and color to 

 adapt them to securing the dispersal of the pollen by various 

 agents, especially insects. Such a shoot bearing stamens, 

 carpels, and accessory leaves is called s. flower. As a similar 

 aggregation of the spore leaves occurs in horsetails and many 

 club-mosses it is evident that the flower is not distinctive of 

 the seed plants, though it attains the highest specialization 

 among them.j 



* 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 re- 

 lation 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. 



■\ It is for this reason that the term seed plants is preferred to flowering 

 plants. 



