io6 Science of Plant Life 



the process of accumulation becomes one of the most im- 

 portant functions of the stem, and it may include the ac- 

 cumulation of water as well as of food (page 75). 



Stems as photosynthetic organs. In plants with little or 

 no leaf display, the stems may do all or most of the photo- 

 synthetic work. The night-blooming cereus and other cac- 

 tuses, and asparagus and equisetum, have no leaves. The 

 green stems of most herbaceous plants contribute at least a 

 part of the carbohydrates, and the young twigs of many 

 woody plants are green and carry on photosynthetic work. 



Physiologically, then, the stem is an organ that supports 

 and displays leaves to the Hght ; aids reproduction by ele- 

 vating the flowers and seeds ; conducts water to the leaves for 

 transpiration and photosynthesis ; carries food to its own 

 living cells and to those of the roots ; is a place for the tem- 

 porary accumulation of food materials ; and may carry on 

 photosynthesis. 



External features of woody stems. On a woody stem nodes, 

 leaf scars, buds, and lenticels may be seen. The nodes are the 

 places where the leaves arise, and they are the most promiaent 

 external feature of stems. The arrangement of leaves at the 

 nodes has already been discussed (page 37). In addition to 

 the leaf, the node gives rise to one or more buds, just above 

 the place of leaf attachment, in the so-called axil (Latin : 

 axilla, armpit) of the leaf. The leaf scars are markings on the 

 stem where leaves have fallen. The part of a stem between 

 two nodes is called an internode. The lenticels are small, 

 dotlike elevations scattered over the surfaces of the internodes. 



Buds. Stems and branches produce leaves only once. We 

 are accustomed to speak of deciduous trees clothing themselves 

 with a new set of leaves each spring, as though the branches 



