THE LEAF 39 
raphides are very irritating and seem to afford some protection 
from animals, although many plants with raphides are eaten by. 
animals. At least in some cases the pain produced is due to the 
entrance of an irritating substance into the wound rather than to 
the raphides themselves. Raphides are destroyed by boiling, and 
so food plants containing them are not irritating when cooked. 

Fig. 24. Cross section of a midrib of a leaf (/zora) 
On the left is a portion of the thin part of the blade, showing upper and lower 
epidermis and chlorenchyma. e, epidermis; ¢, collenchyma; p, parenchyma ; 
s, sclerenchyma ; ph, phloém ; x, xylem. (x 185) 
The midrib. The functions of the midrib are to strengthen 
the leaf; to convey food, manufactured in the leaf, toward the 
petiole; and to carry water from the petiole to various parts of 
the blade. The midrib is composed of the tissues described in 
the following paragraphs. <A tsswe is a group of contiguous 
cells having the same general characteristics. Cross sections of 
midribs are shown in Figs. 24 and 26. 
