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 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 

 second leaf (passing the shorter way) is 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 

 turm ; M the sixth would come directly before, or within, the first. 



