DISTRIBUTION OF LEAVES ON THE STEM. 



123 



less common, though not rare in monocotyledonous plants. Fig. 

 240 illustrates it in a Sedge, and 241 is a diagram in horizontal 

 section, as of a bud ; both 

 extending to six leaves or 

 two turns of the spiral. The 

 fraction J- designates this 

 arrangement. The angular 

 divergence, or distance of 

 the axis of the first leaf 

 from the second, and so on, 

 is one third of the circum- 

 ference (or 120) : conse- 

 quently the fourth leaf comes 

 over the first, the fifth over 

 the second, the sixth over 

 the third, and so on ; that 



\ 



is, the leaves fall into three 

 vertical ranks. The spiral 

 character here begins to be 

 manifest, or becomes so by 

 drawing a line on either fig- 

 ure from the axis or midrib 

 of the first leaf to that of 

 the second, and so on to the 

 sixth, forming a helix of 

 two turns. 1 



Pentastichous, or Five-ranked, sometimes termed the quincuncial 

 arrangement. This is the most common in alternate-leaved 

 dicotyledonous plants. It is shown in Fig. 236 (on a branch 

 of Apple-tree) , and by diagrams, displaying the spiral character, 

 in Fig. 242, 243. The angular distance from the first to the 



O O 



second leaf (passing the shorter way) is f of the circumference, 

 or 144. But the spiral line makes two turns round the stem, 

 on which six leaves are laid down, with angular divergence of f , 



1 The line is supposed to follow the nearest way, and the divergence is 

 counted as , this being the simplest and most convenient. If for any reason 

 the longer way is preferred, then the angular divergence would be expressed 

 by the fraction |. 



FIG. 240. Piece of a stalk, with the sheathing bases of the leaves, of a Sedge-Grass 

 (Carex crus-corvi), showing the three-ranked arrangement. 241. Diagram of the cross- 

 section of the same. The leaves are numbered in succession. 



FIG. 242. Diagram of position of six leaves in the five-ranked arrangement : a spiral 

 line is drawn ascending the stem and passing through the successive scars which mark 

 the position of the leaves from 1 to 6. It is made a dotted line where it passes on the 

 opposite side of the stem, and the scars 2 and 5, which fall on that side, are made 

 fainter. 243. A plane horizontal projection of the same; the dotted line passing from 

 the edge of the first leaf to the second, and so on to the fifth leaf, which completes the 

 frura; as the sixth would come directly before, or within, the first. 



