44 



The leaf 



81-82. — Five-ranked arrangement ; 

 81, vertical diagram; 82, horizontal tioncd should 

 diagram. 



nate or spiral arrangement are the three-ranked (Figs. 79 

 and 80), in which three leaves are passed in completing a 

 turn round the stem, the fourth in vertical order 

 standing over the first; and the five-ranked 

 (Figs. 81 and 82), in which five leaves are passed 

 in making two turns, and the sixth in numerical 

 order stands above the first. This is the com- 

 monest of all the modes of insertion, and the 

 one that prevails among our 

 forest trees and shrubs. The 

 two-ranked is characteristic 

 of the grass family, and the 

 three-ranked of the sedges, 

 though both occur among 

 other plants as well. Speci- 

 mens of all the kinds men- 

 be examined 

 and compared with the dia- 

 grams. There are other and more complicated arrange- 

 ments, but they are not common enough to demand 

 attention here. 



53. Relation between Phyllo- 

 taxy and the Shape of Leaves. 



— Compare the vertical distance 

 between leaves on the same and 

 on different twigs ; are the inter- 

 nodes all of the same length .' 

 Where the internodes are short, 

 the leaves will be crowded to- 

 gether in closer vertical rows. 



A compact arrangement of this 83. — Narrow leaves in crowded 



sort tends to shut off light from ^e.r\\c& rows, 



the lower leaves ; hence, in plants where it prevails, the 

 leaves are apt to be long and narrow in proportion to 

 the frequency of the vertical rows. The yucca, oleander, 

 Canada fleabane, and bitterweed {Helenijnn tenuifolium), 

 all illustrate this law. 



