AERIAL STEMS 151 



account of this twisting of the stem, usually arise from a 

 different side from those at the neighboring nodes ; it is a 

 different side which is upper. (See Figure 50.) 



Evidently it is a disadvantage to prostrate stems to 

 have leaves arising first from one side of the stem and 

 then from the other ; the stem must twist to overcome this 

 disadvantage. Erect stems, however, have the same habit 

 and the advantage to them is apparent. Erect stems are 



FIG. 50. A prostrate stem showing the way it twists, thus helping the leaves in 

 attaining a favorable relation to light. 



not twisted, but adjoining nodes give rise to leaves on 

 different sides, and this is evidently a good device for the 

 avoidance of the shading of one set of leaves by the set 

 next above it. (See Figures i and 5.) In prostrate stems 

 we seem to have in this matter a case of an inherited habit 

 which is of no advantage. It suggests that the ancestors 

 of prostrate plants with twisted stems may themselves 

 have been erect. 



C. Climbing Stems. In the introductory part of this 

 chapter you read of the struggle of plants for light, es- 

 pecially in forests. It is in forests, and apparently it is 



