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 phyllosiphonic, as contrasted with the other groups of 
pteridophytes, which are cladosiphonic, meaning that they have branch 
FIG. 378. The protostele of Gleichcnia: 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 (figs. 383, 386) and circinate 
