t6o 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). 



FlG. 379. The amphiphloic siphonostele of Adiantum: 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 



