DIVISION III. 



PTERIDOPHYTA (VASCULAR 

 CRYPTOGAMS). 



The alternation of generations is as distinct in this Division as 

 in the Mosses, but the sexual generation consists of only a small 

 thallus, the prothallium, which bears directly the sexual organs, 

 antheridia and archegonia ; and the asexual generation, which 

 arises from the fertilisation of the oosphere, is no longer a single 

 short-lived sporangium, but a highly developed, generally per- 

 ennial, plant provided with stem, leaves and true roots (Ferns, 

 Horsetails, etc.), the sporangia being borne on the leaves. In this 

 latter generation the tissues are differentiated into epidermis, 

 ground tissue and vascular tissue; in the last named the bundles 

 are closed, and in the majority of cases concentric. 



The sexual generation, gametophyte, or prothallium, is 

 always a thallns, although not always green and leaf-like (Figs. 



205, 215, 222, 229, 235, etc.) It is very small, even in cases where 

 it attains the greatest development, and consists only of paren- 

 chymatous cells. The prothallium is nourished by hair-like roots 

 (rhizoids) and has only a transitory existence, dying soon after the 

 fertilisation of its oosphere. 



The ANTHERIDIA exhibit great variations in structure which, 

 however, must be considered as modifications of the fundamental 

 type which is found in the Mosses. These modifications will be 

 mentioned under the various families. The spermatozoids are 

 always spirally-coiled, self-motile, protoplasmic bodies, with most 

 frequently a large number of fine cilia on the anterior end (Figs. 



206, 223, 234). They are formed principally from the nucleus of the 

 mother-cell, and portions of the cytoplasm often remain for a 

 time attached to their posterior end. 



The ARCHEGONIA are more uniform throughout the entire 

 Division, and more closely resemble those of the Mosses. They 

 are, as in the previous Division, principally flask-shaped ; but the 



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