The Stem. 81 



growing points (buds 127) are normally formed, and 

 whence roots usually start first in cuttings and layers 

 (358, 349). 



116. The Stem Lengthens by Elongation of the In- 

 ternodes, as well as by the formation of new ones. As 

 the internodes soon attain ' their ultimate length, it 

 follows that the stem lengthens only near its distal end. 

 An internode that has once ceased elongating does not 

 usually resume it, hence the internodes of per- 

 ennial plants that are only partly elongated at 

 the close of the growing season in general re- 

 main undeveloped. When growth is resumed 

 in spring, the formation of a comparatively long 

 internode beyond the very short ones of au- 

 tumn usually forms a perceptible ring about 

 the shoot, which enables us to readily locate the 

 point at which growth started in the spring 

 (Fig. 36). Indeed we can often determine the %. f 

 amount of growth that, took place during the 

 preceding season or even farther back. 



117. The Ultimate Length of the Internodes fig. se 



° Union of 



in any plant, or any part of a plant, depends "^^^fJJ,^ 

 upon the rate of growth— rapid growth pro- wooa. 

 ducing long internodes, and vice versa. In the same 

 species, therefore, the average length of the internodes 

 is much greater in vigorous young plants than in old 

 ones; in the main, central shoot than in the branches, 

 and when growth is well started in spring than during 

 its decline in autumn. The diameter of young inter- 

 nodes that are not unduly shaded is generally in pro- 

 portion to their length, hence rapidly-growing shoots 



