vii.] THE STEM. 61 



The internal structure of the stem may be more suitably 

 described when we speak of cells and tissues. 



4. LEAVES. We have already spoken of leaves as 

 originating successively, either singly or in pairs or in 

 whorls, around the growing apex of the stem, as minute 

 cellular projections. They are never terminal organs ; 

 neither are they, normally, capable of forming buds upon 

 their surface. When a single leaf is given off at each node, 

 the leaves are said to be alternate; if a pair of opposite 

 leaves, they are described as opposite j if three or more in 

 a whorl, as ver titillate. The arrangement of the foliage- 

 leaves upon the stem, though at first sight it may appear 

 accidental, is according to a generally constant law in 

 the same kind of plant. Compare, with respect to leaf 

 arrangement, a young shoot of Oak or Apple with one 

 of the Strawberry-tree (Arbutus) or Monkshood. Try 

 to find two leaves exactly or nearly in the same straight 

 line, one above the other. In the Oak or the Apple 

 the upper leaf will usually be removed from the lower 

 by five internodes ; in the Strawberry-tree or Aconite, 

 by eight. 



The general habit of the plant, as well as the arrange- 

 ment of the leaves, is necessarily affected by the extent to 

 which the internodes of the stem develop. In Buttercup 

 we find the lower leaves springing in a tuft from the short 

 stock, owing to the non-development of the lower inter- 

 nodes while the upper leaves are separated from each 

 other by distinct, and often long, internodes. A parallel 

 but more remarkable case we see in American Aloe 

 (Agave] and Adam's Needle ( Yucca\ and, on a small 

 scale, in the Lettuce and Dandelion, in which plants a 

 succession of (really alternate) leaves are given off from a 

 very short stem or stock, the internodes of which are not 

 perceptibly developed. This is continued until the ap- 

 proach of the flowering season, when the stem suddenly 

 begins to lengthen out, and the leaves either cease alto- 

 gether for a long interval, as in Daisy and Dandelion, or 

 gradually decrease in size to mere scales, as in American 

 Aloe. In our deciduous trees, the nodes from which 

 foliage-leaves are given off are developed at tolerably 

 uniform intervals, but in Pine, Larch, and Cedar an 

 exceptional condition occurs. In the Pines there are 



