72 



ARRANGEMENT OF LEAVES ON THE STEM [LESSON 10. 



I 



appear to be so, are always the leaves of a whole branch which 

 remains so very short that they are all crowded together in a 

 bundle or rosette ; as in the spring leaves of the Barberry and of 

 the Larch (Fig. 139). In these cases an examination shows them 

 to be nothing else than alternate leaves, very much crowded on a 

 short spur ; and some of these spurs are seen in the course of the 

 season to lengthen into ordinary shoots with scattered alternate 

 leaves. So, likewise, each cluster of two or three needle-shaped 

 leaves in Pitch Pines (as in Fig. 140), or of five leaves 

 in White Pine, answers to a similar, extremely short 

 branch, springing from the axil of a thin and slender 

 scale, which represents a leaf of the main shoot. For 

 Pines produce two kinds of leaves; 1. primary, the 

 proper leaves of the shoots, not as foliage, but in the 

 shape of delicate scales in spring, which soon fall away ; 

 and 2. secondary, the fascicled leaves, from buds in the 

 axils of the former, and these form the actual foliage. 



184. Spiral Arrangement of Leaves, If we examine any i\\ 



alternate-leaved stem, we shall find that the leaves are 

 placed upon it in symmetrical order, and in a way per- 

 fectly uniform for each species, but different in different 

 plants. 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 have just the 

 same number of leaves upon it for each turn round the 

 stem. That is, any two successive leaves will always 

 be separated from each other by just an equal portion 

 of the circumference 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 each 

 leaf; 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. 



185. The greatest possible divergence 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 



FIG. 140. Piece of a branchlet of Pitch Pine, with three leaves in a fascicle or bundle, in 

 the axil of a thin scale which answers to a primary leaf. The bundle is surrounded at the 

 base by a short sheath, formed of the delicate scales of the axillary bud. 



