STEMS 



59 



The characteristic sugar has been obtained by the sap from 

 food stored in the stem, notably in the older wood. 



26. Growth in length. Growth in length begins at the 

 tip of the stem by the formation of new cells, which are 

 organized into alternating nodes and 



internodes. When these regions are 

 first formed the internodes are very 

 short, and their subsequent elonga- 

 tion, separating the nodes, is the chief 

 cause of the lengthening of the stem. 

 Internodes are able to elongate for 

 only a certain time, so that the elon- 

 gating portion of a stem does not often 

 extend more than ten to twenty inches 

 below the tip. Seedlings such as those 

 of the bean should be cultivated, and 

 the region of growth, the region of 

 greatest growth, and the rate of growth 

 determined. The same method may 

 be used as was used with the leaf 

 ( 16), in this case each internode 

 being marked with equally spaced lines 

 in India ink. Measuring these spaces 

 at intervals of one or two days will 

 determine the facts referred to above 

 (Fig. 57). 



27. Special forms of stems. Usu- 

 ally branches resemble the stem from 

 which they arise, but occasionally they 



differ in a striking way. That these different structures are 

 really branches is usually evident to external observation 

 from the fact that they stand in the position of branches, 

 that is, in the axils of leaves ( 22). The three following 

 forms illustrate axillary structures that do not resemble 

 ordinary branches. 



FIG. 57. Scarlet runner 

 bean marked with a 

 scale of five millimeter 

 intervals and photo- 

 graphed after forty- 

 eight hours ; the lines 

 closest together show 

 the original spacing. 



