124 PLAN 7^ LIFE. 



exist between such forms and those that are nuieh folded or 

 crumpled, tliiek and flesliy, or even cylindrieal. 



Fig. 144. — A shoot of Acacia, slinwinL; at <; a twice-branched (compound) leat with 

 roundish petiole ; at /', a similar leal with tlattcncd Ijlade-like petiole ; at , , phyllodia, 

 i.e., blade-Hke petioles without true blades About hall natural size (:').— Alter'Frank. 



If a thin blade be held between the eye and the liylit, two 

 parts become evident : ( i ) a green tissue (mesophyll), more 

 or less opa(|ne ; and{2) translucent "nerves" or"\-eins."* 

 The larger of these, usually called tlie " ribs," * frequently 

 form ridges ufjon the under surface, f 



155. Branching. — 'I'he outline of the blade is extremely 

 various. It is dependent upon the character and extent of 

 its branching, which may be either slight or extensi\'e. 

 Slight branching gives rise to teeth of various forms (fig. 



* These words must not be thought to indicate any resemblance in func- 

 tion to tlie same parts in animals, but only similarity of position or ap- 

 pearance. 



f For further account of structure see \ 168. 



