44 



THE STORY OF THE PLANTS. 



in great groups of plants closely related by 

 descent to one another. The innnense difference 

 in their external shape (which often varies enor- 

 mously even on the same stem) is mainly due to 

 the relative extent to which the framework is 

 filled out or not with living cell-stuff, or, as it is 

 technically called, cellular tissue. 



There are two chief ways of arranging the ribs 

 or veins in a leaf, which may be distinguished as 

 the fuKjcr-like and the feather-like methods (in 



FIG, 2.— FiNGKR-YEiNED LEAVKS. The veiiis arc the same 

 in the three leaves, but they differ in the amount to 

 which tliey are filled in. 



technical language, j)aZ??ir/i6 and _2;i?ina^e). In 

 the finger-like plan the ribs all diverge from a 

 common point, more or less radially. In the 

 feather-like plan the ribs are arranged in oppo- 

 site pairs along the sides of a common line or 

 midrib. Yet even these two distinct plans 

 merge into one another by imperceptible de- 

 grees, as you can see if you look at the accom- 

 panying diagram. 



Now let us take first the finger -veined type 

 (Fig. 2). Here, if all the interstices of the ribs 

 are fully filled out with cellular tissue, we get a 



