44 



THE LEAF 



81-82. Five-ranked arrangement : 

 8 1, vertical diagram; 82, horizontal 

 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. 8 1 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- 

 tioned should 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 vertical 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 (Helenium tenuifolium\ 



all illustrate this law. 



