42 



THE STORY OF THE PLANTS. 



plan the ribs are arranged in opposite pairs along 

 the sides of a common line or midrib. Yet even 

 these two distinct plans merge into one another 

 by imperceptible degrees, as you can see if you 

 look at the accompanying 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 

 roundish leaf like that of the so-called nastur- 

 tium. But if the ribs project a little at the edge 

 — in other words, if the cellular tissue does not 



Fig. 3. — Feather-veined leaves. The four leaves have similar 

 veins, but are differently filled in. 



quite fill out the whole space between them — we 

 get a slightly indented leaf, like that of the scar- 

 let geranium or the common mallow. If the un- 

 filled spaces between the ends of the ribs are 

 much greater, then the ribs project into marked 

 points or lobes, and we get a leaf like that of ivy. 

 Carry the starving of the cellular tissue a little 

 further still, and we have a deeply-indented leaf 

 like that of the castor-oil plant. Finally, let the 

 spaces unfilled go right down to the common 

 centre from which the ribs radiate, and we get a 



