SECTION 7.] 



THEIR ARRANGEMENT. 



G9 



185. Phyllotaxy of Alternate Leaves. Alternate leaves are distrib- 

 uted along the stem in an order which is uniform for each species. The 

 arrangement in all its modifications is said to be spiral, because, if we 

 draw a line from the insertion (i. e. the point of attachment) of one leaf to 

 that of the next, and so on, this line will wind spirally around the stem as 

 it rises, and in the same species will always bear the same number of leaves 

 for each turn round the stem. That is, any two successive leaves will 

 always be separated from each other by an equal portion of the circum- 

 ference of the stem. The distance in height between any two leaves may 

 vary greatly, even on the same shoot, for that depends upon the length of 

 the internodes, or spaces between the leaves ; but the distance as measured 

 around the circumference (in other words, the Angular Divergence, or angle 

 formed by any two successive leaves) is uniformly the same. 



186. Two-ranked. The greatest possible di- 

 vergence is, of course, where the second leaf stands 

 on exactly the opposite side of the stem from the 

 first, the third on the side opposite the second, and 

 therefore over the first, and the fourth over the 

 second. This brings all the leaves into two ranks, 

 one on one side of the stem and one on the other, 

 and is therefore called the. Two-ranked arrangement. 

 It occurs in all Grasses, in Indian Corn, for in- 

 stance ; also, in the Basswood (Fig. 181). This 

 is the simplest of all arrangements, and the one 

 which most widely distributes successive leaves, but 

 which therefore gives the fewest vertical ranks. 

 Next is the 



187. Three-ranked arrangement, that of all 

 Sedges, and of White Hellebore. Here the second 

 leaf is placed one third of the way round the stem, 

 the third leaf two thirds of the way round, the fourth 

 leaf accordingly directly over the first, the fifth over 

 the second, and so on. That is, three leaves occur 

 in each turn round the stem, and they are separated 

 from each other by one third of the circumference. 

 (Fig. 186, 187.) 



188. Five-ranked is the next in the series, and 

 the most common. It is seen in the Apple (Fig. 188), Cherry, Poplar, 

 and the greater number of trees and shrubs. In this case the line traced 

 from leaf to leaf will pass twice round the stem before it reaches a leaf 



_... 4 



'. 2 



Fig. 186. Three-ranked arrangement, shown in a piece of the stalk of a Sedge, 

 with the leaves cut off above their bases ; the leaves are numbered in order, from 

 ltofi. 187. Diagram or cross-section of the same, in one plane ; the leaves simi- 

 larly numbered ; showing two cycles of three. 



