114 PLANTS, THE FOOD OF ANIMALS 



The Leaf or Frond. The aerial part of Pteris consists of amain 

 branch or stem arising as an outgrowth from the rhizome. 

 The branches, termed pinnae, in turn give rise to the flattened 

 chlorophyll-bearing structures analogous to leaves of higher 

 plants but here termed pinnules. (In the comparative mor- 

 phology of plants analogous parts do not always bear the same 

 names, thus the leaf of the fern is the entire aerial part of the 



FIG. 47. Cross section of a portion of a leaf showing the epidermis (tp.) 

 the palisade mesophyil (above) and the spongy mesophyfl (below). Sections 

 through the guard ceDs and stomata (si.} show the openings into the inter-cellu- 

 lar spaces (.). (From Sedgwick and Wilson.) 



plant while the rhizome corresponds to the trunk of a tree. 

 The trunk is underground, the leaves alone being exposed.) 

 The stem or frond is somewhat thickened at the point where it 

 enters the ground, thus giving greater resistance to wind, etc. 

 It is generally supposed that the frond is only a much thinned 

 or flattened outgrowth of the stipe or stem. It is composed 

 of the same series of cells as those found in the rhizome but some 



