THE PTERIDOPJIYTA 333 



appear when tlu? piothallu.s is fully grown and considerably after 

 the antheridia have liberated their sperms. At maturity, the 

 neck of the archegonium opens and the neck-canal cells break 

 down, producing a substance attractive to the sperms. The 

 antheridium is also very much smaller and simpler than it is 

 among bryophytes. Its wall consists of but three cells — a cover- 

 cell, a circular cell which forms the main wall, and a funnel- 

 shaped basal cell. The contents of the antheridium divides into 

 a large number of sperms, each possessing a tuft of cilia by which 

 it can swim about in a thin film of water (Fig. 203). A sperm 

 enters an archegonium and there effects fertilization, after which 

 the fertilized egg, by repeated cell divisions, forms a mass of 

 tissue which gradually becomes differentiated into the body of the 

 young sporophyte. This soon develops a vigorous root and 

 shoot (Fig. 204) and grows into the mature fern plant. The life- 

 cycle of a fern is graphically represented in Fig. 205. 



The order includes nearly 3,000 species and its members 

 are widely distributed over the globe, being particularly rich in 

 species and individuals throughout all tropical regions. It 

 is by far the largest group of pteridophytes and since early 

 times has occupied an important place in the earth's vegetation. 



2. Ophioglossales or Adder's Tongues. — Here are placed a small 

 group of fern-like plants which are of interest to botanists from 

 their possession of certain characteristics markedly different 

 from those of other ferns. A single leaf, simple in the adder's 

 tongue fern but typically fern-like in the rest of the order, is 

 produced each season by the subterranean stem. From the 

 petiole of this leaf arises a spore-bearing stalk crowned with a 

 cluster of heavy-walled sporangia, very different in type from the 

 thin-walled structures of true ferns. The gametophyte is thick 

 and tuberous, and is partly subterranean. The Ophioglossales 

 are often placed by themselves in a separate class. 



3. Hydropteridales or Water Ferns. — This is another small 

 group, chiefly important for its specialized method of reproduc- 

 tion. Its sporophyte is aquatic and bears little or no resemblance 

 to that of the true ferns. Marsilia (Fig. 206), the clover-leaf 

 fern, is the commonest representative. From the petiole of its 

 curious four-lobed leaf arise one or more bean-like sporocarps 

 containing sporangia of two soYts;t\ie megasporangia, each produc- 

 ing a single large megaspore, and the microsporangia, each 

 producing a group of smaller microspores. These are dispersed in 



