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. so. — 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 



