194 ARRANGEMENT OP LEAVES UPON THE AXIS. 



in which this is done, varies in different tribes of plants. Any 

 one may examine it, with the certainty of finding what will 

 greatly interest him, by cutting across the leaf-buds with a sharp 

 knife, when they are swelling, but before they have begun to 

 expand. The outer scales are sometimes covered with a thick 

 down, which may serve as a protection to them against the 

 cold ; and sometimes they are coated over with a gluey sub- 

 stance, as in the horse-chesnut, which seems a very efficient 

 guard. 



302. The young leaves, in most leaf-buds, may be easily ob- 

 served to be arranged around a common centre or axis. When 

 the bud lengthens, the insertions of the leaves, which were at 

 first close together, are separated by the lengthening of the branch 

 which bears them ; and they then generally assume something 

 of a spiral or rather a corkscrew-like arrangement round it, which 

 is often very apparent. In fact this may be regarded as the 

 regular mode in which leaves are arranged upon any part of the 

 stem or branches of a tree. Starting from any one leaf, we 

 shall generally find the next leaf not exactly above or below that 

 one, but a little to one side of the perpendicular ; the next a 

 little to one side of that ; and so on, until we come directly 

 over the one from which we set off. We shall have thus made a 

 spiral round the stem ; and the number of leaves we meet with 

 in its course, varies in different species of plants. Sometimes it 

 amounts to twenty or more. Sometimes we only find two ; in 

 this case each leaf is nearly on the opposite side of the stem from 

 the other, but higher up or lower down. Leaves are said in all 

 these cases to be alternate. The point of the stem from which a 

 leaf originates, is called a node; and the space between two 

 nodes is called an internode. 



303. Now although it may be considered as the regular kind 

 of growth, for a branch to lengthen equally throughout, yet we 

 not unfrequently meet with varieties in the arrangement of 

 leaves, occasioned by the cessation of growth at particular points. 

 Thvs, if the internode between any two alternate leaves is not 

 developed, the leaves will be opposite to each other. Again, 

 where each spiral turn contains several leaves, if all the inter- 



