GENERAL STRUCTURE OF STEMS 



183 



strengthening tissues assume a tube-like arrangement, which is well 

 known to engineers as the arrangement in which the most strength 

 with a given amount of material can be secured. The truth of 

 this principle is demonstrated in the construction of bicycle 

 frames, where much strength with little weight is secured by using 

 large tubes instead of solid rods. Again, having the cylindrical 

 form, stems can be equally resistant to strains from all directions. 

 Stems taper and also decrease in age from the base of the trunk 

 to the end of the twigs where 

 the stem tissues are in process 

 of formation from the apical 

 meristems. The apical meri- 

 stems are also known as pri- 

 mary meristems because they 

 form other meristems, notably 

 the cambiums. It is on the 

 new elongation at the tips of 

 the stem, that the leaves appear 

 anew each year. The nodes, 

 the regions where leaves and 

 buds occur, are separated by 

 the elongation of the inter- 

 nodes, and in this way the 

 leaves, which are younger and 

 more crowded the nearer the 

 tip, are separated and exposed 

 to the Kght. In most annual 

 stems the nodes are all formed 

 very early, and elongation 

 thereafter consists chiefly in 

 the lengthening of the internodes, which thereby separate the 

 leaves so that they can unfold and expand to their mature size. 

 Thus, as shown in Figure 159, the nodes and internodes of a Com 

 stem are all present in a Corn seedling two or three weeks old. 



In most herbaceous stems, where there is no need of corky bark 

 and almost the entire stem is leaf bearing, the stem is active 

 throughout in the manufacture of food. But in perennials, such as 

 shrubs and trees, photosynthesis is limited to the young branches 

 where the leaves are borne and the light is not excluded from the 

 green cortex of the branches by a corky covering. In passing 



Fig. 159. — Lengthwise section 

 through the stem of a Corn seedhng, 

 showing the apical meristems (m), the 

 nodes (ra), and the short internodes (i). 



