116 



PLANTS, THE FOOD OF ANIMALS 



Its 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 

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

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



ep* 



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

 the palisade mesophyll (above) and the spongy mesophyll (below). Sections 

 through the guard cells and stomata (st.) show the openings into the inter-cellu- 

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



The stalk of the 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 of them have undergone modifications. The epidermis 

 cells of the rhizome are lifeless, but here they are living and 

 elaborated into flattened epidermal cells with curious wavy out- 



