IX] PTERIDOPHYTA. 243 



that he should make himself familiar with the rarer, less known 

 and more isolated recent forms, which often throw most light on 

 the affinities of the older representatives of the group. It is 

 often the case, the more isolated living plants are, the more 

 likely are they to afford valuable assistance in the inter- 

 pretation of genera representing a class, which reached its 

 maximum development in the earlier periods of the earth's 

 history. The importance of paying special attention to such 

 recent plants as may be looked upon as survivals of a class now- 

 tending towards extinction, will be more thoroughly realised 

 after the extinct vascular cryptogams have been dealt with. 



A comparison of the Pteridophyta and Bryophyta brings 

 out certain points of divergence. In the first place, the sporo- 

 phyte assumes in the former class a much more prominent 

 role, and the gametophyte has suffered very considerable re- 

 duction. The gametophyte, i.e. the structure which is formed 

 on the germination of the asexual ly-produced spore, is usually 

 short-lived, small, and more or less dependent on the sporo- 

 phyte for its nutrition. In a few cases only is it capable of 

 providing itself with the essential elements of food. On the 

 other hand, the sporophyte, at a very early stage of its develop- 

 ment becomes free from the gametophyte and is entirely self- 

 supporting. Reproduction is effected as in the Bryophyta by 

 sexual reproductive organs and by asexual methods. Not only 

 have we in the Pteridophytes a much more complete external 

 division of the plant-body into definite members, which subserve 

 distinct functions, and behave as well-defined physiological 

 organs adapted for taking a certain share in the life-functions 

 of the individual, but the internal differentiation has reached 

 a much higher stage. True vascular tissue, consisting of xylem 

 and phloem, occurs for the first time in this class. The whole 

 plant is traversed by one or more vascular strands composed 

 of xylem and phloem elements, which are respectively con- 

 cerned with the distribution of inorganic and organic food 

 substances. 



The Pteridophyta include the most important fossil plants. 

 It is from a study of the internal structure of various extinct 

 representatives of this class, that palaeobotanists have been 



16—2 



