SUMMARY OF THE LIFE HISTORY OF A FERN 319 



the root and leaf are well established the sporophyte becomes 

 independent of the gametophyte, which gradually dies within 

 a few weeks or months. It is the development of root, stem, 

 and leaf on the part of the sporophyte, giving it complete inde- 

 pendence, which marks the greatest advance of the pteridophytes 

 over the bryophytes. 



310. Summary of the life history of a fern. The alterna- 

 tion of generations in the fern is much more apparent than in 

 the liverworts and mosses because both gametophyte and sporo- 

 phyte are independent plants. The two groups (bryophytes and 

 pteridophytes) are in striking contrast in the relative importance 

 of the two generations. The gametophytes of the bryophytes 

 are relatively large, long-lived, and complex organisms (with 

 stems and leaves in the mosses and leafy liverworts), while the 

 sporophytes are simple and so dependent upon the gametophyte 

 that they were for many years called its fruit. The gameto- 

 phytes of the pteridophytes, on the contrary, are small, short- 

 lived, and simple, while the sporophytes are very large and 

 complex (possessing stem, roots, leaves, and a vascular system) 

 and, except in their earliest stages of development, completely 

 independent of the gametophytes. 



The life history of a fern may be formulated as follows : 



Gametoplii/ie (prothallium) < -Xporophylc (fern plant) 



^99 



- asexual spores Gametophyte, etc. 

 This in abbreviated form becomes 



->S-sp - G, etc., 



and is the same life-history formula as that of the bryophytes 

 (Sec. 285). 



311. Apogamy and apospory in the ferns. There are some 

 irregularities in the life histories of certain ferns which are 

 not uncommon in greenhouses and under cultivation (species 

 of Pteris, Aspidium, Athyrium, Nephrodium, etc.). Prothallia 



