THE PLANT-BODY 



31 



-O 



cells of these lower plants are naked, 

 often motile cells (spores) formed in 

 special structures, sporangia, from 

 which they are set free, and germi- 

 nate immediately (Fig. 23). 



Gametes. Sexual cells, or Gam- 

 etes, are also found, often closely 

 resembling the non-sexual spores, 

 from which they differ in not being 

 capable of independent growth. Two 

 of these gametes must unite to pro- 

 duce the germ of the new plant. One 

 of the gametes is usually much larger 

 than the other, and is retained within 

 the organ where it was formed ; the 



smaller gamete, the male or sperm- 



. . ,! i FlG - 24. A, qpgomum of (Edogo- 



cell, is often actively motile and nium BoscU (x 200). B, arche- 

 Swims to the female cell, with which gonium of Madotheca platyphylla 

 it fuses. The gametes are borne in (*300); o, the egg. 

 structures much. like the sporangia, but which sometimes show con- 

 siderable complexity (Fig. 24). 



Alternation of Generations. 

 Among the higher plants there is 

 a marked difference between the 

 plants which produce the sexual 

 and those which bear the non- 

 sexual reproductive cells. The 

 sexual generation is known as the 

 " Gametophyte," the non-sexual as 

 the " Sporophyte." There is an 

 increasing tendency among these 

 plants toward the suppression of 

 the sexual phase, which becomes 

 excessively reduced in the Flower- 

 ing Plants, where the sporophyte 

 is the plant as we ordinarily recog- 

 nize it. 



The spores of the Ferns and 

 Flowering Plants are produced in 

 characteristic sporangia which most 

 commonly are outgrowths of the 

 special leaves, or Sporophylls (Fig. 

 25). The spores germinate at once 

 and produce the gametophyte. 

 The sporangia of the Ferns are 



B 



FIG. 25. A, sporophyll of Osmunda 

 C'laytoniana sp, fertile leaf-seg- 

 ments. B, sporophylls of Equi- 

 setum, arranged in a cone at the 

 apex of the shoot. 



