124 



PLANT LIFE. 



exist between such forms and those that are much folded or 

 crumpled, thick and fleshy, or even cylindrical. 



FIG. 144. A shoot of Acacia, showing at a a twice-branched (compound) leaf with 

 roundish petiole ; at b, a similar leaf with flattened blade-like petiole ; at c, phyllodia, 

 i.e., blade-like petioles without true blades. About half natural size (?). After Frank. 



If a thin blade be held between the eve and the li< r ht, two 



j O / 



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

 or less opaque; and (2) translucent "nerves" or "veins." * 

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

 form ridges upon the under surface, f 



155. Branching.- -The outline of the blade is extremely 

 various. It is dependent upon the character and extent of 

 its branching, which may be either slight or extensive. 

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



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

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

 pearance. 



f For further account of structure see \ 168. 



