THE LEAVES. 133 



former between the liber and the wood, and therefore no provision 

 for increase in diameter. The bundles are therefore strictly limited, 

 while those of Exogens are unlimited in growth. In Exogens the 

 woody bundles or wedges, symmetrically arranged in a circle, be- 

 come confluent into a zone in all woody and most herbaceous stems, 

 which continues to increase in thickness. In Endogens the woody 

 bundles are unchanged in size after their formation, but new and 

 distinct ones are formed in the growing stem with each leaf it de- 

 velops, and interspersed more or less irregularly among the older 

 bundles. 



CHAPTER V. 



OP THE LEAVES. 



Sect. I. Their Arrangement. (Phyllotaxis, etc.) 



235. The situation of leaves, as well as their general office in the 

 vegetable economy, and several of their special adaptations, has 

 already been stated. Leaves invariably arise from the nodes (156), 

 just below the point where buds appear. So that wherever a bud 

 or branch is found, a leaf exists, or has existed, either in a perfect or 

 rudimentary state, just beneath it ; and buds (and therefore branch- 

 es), on the other hand, are or may be developed in the axils of all 

 leaves, and do not normally exist in any other situation. The point 

 of attachment of a leaf (or other organ) with the stem is termed its 

 insertion. The subject of the arrangement of leaves on the stem has 

 received the name of 



236. Phyllotaxis (from two Greek words, signifying leaf-arrange- 

 menl). 



237. As to their general position, leaves are either alternate, oppo- 

 site, or verticiUate. They are said to be alternate (127, and Fig. 

 121, 157, 204) when there is only one to each node, in which 

 case the successive leaves are thrown alternately to different sides 

 of the stem. They are said to be opposite when each node bears 

 a pair of leaves, in which case the two leaves always diverge 

 from each other as '^videly as possible, that is, they stand on opposite 



12 



