12 THE MORPHOLOGY OF PTERIDOPHYTES 



time certainly 500 million years and probably as far back as 

 1,000 million (some even say 2,000 million) years. Clearly, 

 the morphologist cannot afford to be a narrow specialist. 

 He must be a biologist in the widest possible sense. 



From taxonomy and paleobotany, the plant morpholo- 

 gist is led naturally to the consideration of the course of 

 evolution of plants (phylogeny), which to many botanists 

 has the greatest fascination of all. However, it must be 

 emphasized that here the morphologist is in the greatest 

 danger of bringing discredit on his subject. His theories are 

 not capable of verification by planned experiments and 

 cannot, therefore, be proved right or wrong. At the best, 

 they can be judged probable or improbable. Theories 

 accepted fifty years ago may have to be abandoned as im- 

 probable today, now that more is known of the fossil record, 

 and, hkewise, theories that are acceptable today may have 

 to be modified or abandoned tomorrow. It is essential, there- 

 fore, that the morphologist should avoid becoming dog- 

 matic if he is ever to arrive at a true understanding of the 

 course of evolution of hving organisms. 



Within the plant kingdom the range of size is enormous, 

 for, on the one hand, there are unicellular algae and bacteria 

 so small that individuals are visible only under the micro- 

 scope, while, on the other hand, there are seed-bearing 

 plants, such as the giant Redwoods of California and the 

 Gums of AustraUa, some of which are probably the largest 

 living organisms that the world has ever known. Accompany- 

 ing this range of size, there is a corresponding range of 

 complexity of internal anatomy and of life-history. Some- 

 where between the two extremes, both in structure and in 

 Hfe-cycle, come the group of plants known as Pteridophytes, 

 for they share with seed plants the possession of well- 

 developed conducting tissues, xylem and phloem, but differ 

 from them in lacking the seed habit. Internally, they are 

 more complex than mosses and Uverworts, yet in life-cycle 

 they differ from them only in matters of degree. 



