CHAPTER EIGHTEEN 



THE GROWTH OF STEMS 



The limit of growth of stems is not so definite as that of leaves. 

 The length and the diameter of a stem depend largely upon the 

 conditions under which the plant lives, the available water supply, 

 amount of light, the length of daylight, the temperature, and 

 quality of the soil. Along a dry roadside a ragweed may complete 

 its development with a stem less than 6 inches long, while in a 

 rich bottom-land field the same plant might have reached a 

 height of 15 feet. 



Growth in length. The growth in length takes place at the 

 apex of a stem, the growing point being located in the terminal 

 bud. The growing region extends back from the tip, sometimes 

 for only a fraction of an inch, more rarely, as in rapidly growing 

 vines, a foot or two. If we mark the upper portion of a growing 

 stem into equal spaces, we may observe on the following day that 

 the uppermost spaces have elongated the most. The adjoining 

 spaces below are less and less elongated. This indicates that the 

 greater part of the cell division takes place near the tip (the grow- 

 ing point), but that some cell division and most of the enlarge- 

 ment of cells occurs in the adjoining part of the stem (the elongat- 

 ing region). During enlargement the minute cells of the growing 

 point absorb water and increase their volume from one hundred 

 to two thousand times. In the growing point the nuclei and the 

 surrounding cytoplasm completely fill the cell walls. In the 

 elongating region the cytoplasm forms merely a thin layer lining 

 the inside of the cell wall, most of the internal space being 

 occupied by cell sap. 



The above description holds for most stems. In grasses and 

 some other monocots, however, the process is slightly modified. 

 In these plants the tissue at or just above each node continues 

 to grow for some time after the tissue of the upper part of the 



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