836 A TEXTBOOK OF THEORETICAL BOTANY 



Morphology of Stems. 



It is often said that a leading character of stems is the formation of joints 

 or nodes, at which the leaves are attached, separated by varying lengths 

 called internodes. 



While this is generally true of the Dicotyledons, it is often otherwise 

 among Monocotyledons, where the broad-based leaves are set so closely in 

 succession that no distinction of nodes is possible. In fact, as we shall see 

 later, this is the primitive condition which we find in the buds of most plants, 

 at the growing points where the leaves are developing. The separation of 

 anv pair of leaves by the elongation of the stem between them is only accom- 

 plished slowly and is closely dependent upon the growth and vigour of the 

 leaves themselves. 



If we examine a straight, well-developed shoot of a plant with opposite 

 leaves, such as that of a Willow Herb, we see that the leaves near the base 

 are small and that the internodes are correspondingly short. The latter 

 increase in length regularly to the level at which the leaves have reached their 

 maximum size and from there they decrease again, in association with 

 immature leaves, until they merge into the apical bud, where they are hidden 

 by the closely placed leaf primordia. On a long shoot, with many leaves 

 fully mature, there will be a corresponding length of stem in which the 

 internode length is roughly constant. It is evident that the growth in length 

 of each internode is to a large extent determined by the nourishment which 

 the leaf immediately above it can supply. In the branches of trees the same 

 sequence of varying lengths of internode occurs independently in each year's 

 growth, beginning at the level of the terminal bud of the previous year, 

 so that each year's growth may be regarded as a distinct " shoot generation." 



The case we first mentioned with opposite leaves is the simplest and 

 clearest in this respect. Where the leaves are single and spirally arranged 

 matters are more complex, for in such stems the internodes overlap and 

 their growth is a composite of effects due to several of the leaves above. 

 This will become clear when apical growth has been studied. 



Although the length of internodes appears to be closely associated with 

 leaf development and does not continue after the leaf has reached full size, 

 it is otherwise with growth in thickness. This is a continuous process, and 

 the diameter of the stem will be found to increase regularly, downwards, as 

 far as the level of the cotyledons, without direct relation to leaf growth. It 

 continues even after the growth in length of the stem as a whole has ceased 

 and in the older parts of the stem, which no longer bear leaves. 



The foregoing is a particular example of what are called symmetry 

 relationships in the plant. The longitudinal symmetry which we have 

 just considered is based upon an imaginary line joining the main stem apex 

 to the main root apex, i.e., the longitudinal axis, which is the chief con- 

 structional line in the architecture of the plant and which we may therefore 

 consider as its principal and probably its oldest dimension. Its outstanding 

 character is its polarity, that is to say, its possession of two opposite and 



