CHAPTER XII 



STEMS 



129. General character. A stem is characterized by 

 bearing leaves, and this involves two problems : (1) the dis- 

 play of leaves, and (2) the conduction of water. Stems are 

 not always rigidly erect, for they may be prostrate or climb- 

 ing ; neither are they always aerial, for many stems are sub- 

 terranean. Often stems are unbranched (simple), in which 

 case the leaf display is relatively small ; often they are 

 branched, which simply means an increased capacity for leaf 

 display, which reaches its maximum in certain types of trees. 



What is called the habit, that is, the general appearance of 

 a plant, is determined by the character of the stem, the leaves 

 being a constant feature in every case. For example, trees 

 are recognized in winter by their stem habit as easily as when 

 they are in foliage. 



130. Leaf display. There are some general facts about 

 the display of leaves by aerial stems that should be recognized. 

 The stem (or branch) of a seed-plant does not produce leaves 

 indiscriminately throughout its whole length, but only at 

 certain definite regions called the nodes, the regions between 

 the nodes being called internodes. This means that the stem 

 is differentiated into two regions, the nodes that produce 

 leaves and the internodes that do not, and these two regions 

 differ also in structure. The significance of this differentia- 

 tion is that while the nodes are constructed to produce leaves, 

 the internodes are constructed to elongate, so that the leaves 

 are separated. Plants vary in the amount of the separation of 



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