l6o MORPHOLOGY 
vernation (fig. 382), which means that the young leaf is enrolled from 
the tip downwards, and in expanding unrolls from the base upwards. 
The internal structure of the leaf is practically the same as that of the 
leaves of seed plants, with epidermis containing stomata, mesophyll 
(both palisade and spongy), and abundant veins (see p. 250). 
FIG. 379. The amphiphloic siphonostele of Adianlum: beginning at the outside 
the general regions of the section are epidermis, a thick cortex, endodermis (the epi- 
dermis-like innermost layer of the cortex which bounds the stele), the outer phloem, the 
xylem, the inner phloem, the inner endodermis, and the pith. 
Sporangia. It will be impossible to describe all the methods of 
sporangium development, but the sporangia of Marattiaceae and Poly- 
podiaceae will be described, with the understanding that the intermediate 
families show intergrading conditions. 
Marattiaceae. The Marattiaceae are eusporangiate, as .are all the 
pteridophytes previously considered, and also the seed plants. Fol- 
lowing the periclinal division of the superficial initial cell, the outer cell 
