PTERIDOPHYTES 159 



Filicales and Ophioglossales, and they seem to be associated with the 

 production of large leaves. On the basis of this character, these groups 

 are said to be phyllosi phonic, as contrasted with the other groups of 

 pteridophytes, which are cladosiphonic, meaning that they have branch 



FlG. 378. — The protostele of Cleichenia: the xylem vessels, recognized by their 

 large caliber and thick walls, occupy the entire central region of the stele; investing 

 them is a zone of sieve vessels (phloem), which in turn are surrounded by the cortex. 



gaps, but no leaf gaps. This distinction has been found to be a very 

 important one in connection with the study of the origin of seed 

 plants. 



Leaves. — The leaves of ferns are the only aerial structures in ordi- 

 nary terrestrial forms. They were formerly called fronds, with the idea 

 that they were not ordinary leaves, but a combination of leaf and stem. 

 They are usually branched, either pinnately or palmately, and are char- 

 acterized by dichotomous (forking) venation (tigs. 383, 386) and circinate 



