CHAPTEE V 



THE STEM 



§ 15. What is the structure of the stem? 



The root passes above the ground into the stem, which pre- 

 sents itself to us as an axis bearing the leaves, or in certain 

 comparatively rare cases, undertaking the function of the leaves 

 itself. 



The external development of the stem axis varies very much 

 in size, duration, and in the composition of its tissues. An 

 axis which only bears flowers is spoken of as a scape, the leaf- 

 bearing axis is called a culm or haulm (in grasses), a stalk (in 

 herbaceous plants), or a stem (in trees). In some plants the 

 main portion of the leaf-bearing axis remains underground, 

 and only the leaves and the tip of the axis are pushed up 

 above the soil. This, however, is not the place to discuss the 

 general morphology and anatomy of the stem, as the physio- 

 logist and gardener are chiefly concerned with the function of 

 the various organs, and their place and importance in the 

 economy of the plant. From this point of view, it is sufiicient 

 for us to remember that we have already termed the leaves 

 the chief seat of formation of organic substance, and to observe 

 that the network of veins which is marked out on the leaf 

 surface collects into considerable bundles which pass down the 

 leaf-stalk and are continued down the stem. 



If we follow the course of these bundles from a young leaf 

 into the stem, we see that it is these leaf-trace bundles which 

 form a ring of bundles in the stem, and which by their further 

 development form the woody cylinder of the older stem. The 

 number of bundles which pass from a leaf into the stem, and 

 which we have already termed the fibro-vascular bundles, varies 

 iii different plants (3, 5, 7, &c.), as can readily be seen from 



