CHAPTER XXVI 



FORMS AND EXTERNAL FEATURES OF STEMS 



<><><><x><><>e<><^<><><x><^^ 



Several facts about stems have already been considered. We have seen 

 that leaves and axillary buds appear on the nodes of stems in a definite 

 arrangement, and that stems of shrubs branch at or near the surface of 

 the ground, in contrast to stems of trees. Foods may accumulate in any 

 stem, but a greater amount accumulates in some stems than in others. 

 The green stems of cacti and man)' herbs and the numerous young 

 twigs of both shrubs and trees contain chlorophyll, and hence photo- 

 synthesis may occur in them. In many processes previously discussed 

 it was evident that sugar made in the leaves moves through the stems 

 to the roots; that water and inorganic salts after diffusing into the roots 

 move to the leaves through the stems, and that processes characteristic 

 of all living cells occur in living cells of stems. Some stems are short- 

 lived; others remain alive and increase in both height and diameter 

 for centuries. We have already seen that the direction and rate of stem 

 growth are affected by gravity, light, moisture, and other environmental 

 factors. In many sections of the country where deciduous forests prevail, 

 the principal features of the forested landscape from late autumn until 

 the early springs are stems. 



What is a stem? The cylindrical, erect, aerial stems of most plants are 

 readilv distinguishable from leaves and roots. Many stems, however, 

 are flattened leaf -like organs; others are succulent, and still others are 

 so short that they are nearly obscured by leafy scales or rosettes of leaves. 

 Many stems are entirely underground. What, then, are the distinguishing 

 characteristics of stems? A stem develops from the plumule (bud) of the 

 embryo, usually bears leaves and flowers, and has certain other charac- 

 teristic features, both external and internal. Examination of several kinds 

 of stems will help us recognize these features. 



Woody stems. Stems 1 to 2 feet long of such plants as hickory, horse- 

 chestnut, or maple (Fig. 82) consist of several yearly increments of 

 growth. The most prominent features of woody stems in winter are the 

 dormant buds, leaf scars, terminal bud scars, and lenticels. Leaves and 



245 



