CHAPTER TWENTY 



ECOLOGICAL TYPES OF STEMS 



Attention has already been called to the variety of stems, 

 and the advantages of upright, horizontal, climbing, and under- 

 ground stems. In a sense these are ecological types of stems, 

 since each of them bears a slightly different relation to the envi- 

 ronment. All the kinds of stems discussed, however, occur among 

 mesophytes, and the descriptions of stem structures that have 

 been given were also based on the stems of plants that live under 

 medium moisture conditions. Since the great bulk of plants live 

 as mesophytes, we may look upon these structural arrange- 

 ments as typical of the plants living under the most favorable 

 circumstances. 



In this chapter the peculiar features of the stems of water 

 plants (hydrophytes) and of desert plants (xerophytes) are de- 

 scribed. Only a comparatively small number of flowering plants 

 are included in these groups. 



How drought modifies mesophytes. When mesophytes are 

 grown under very dry conditions, the stems are reduced in size, 

 the stem tissues are more compact, and the cell walls are heavier. 

 Some plants also develop thorns, spines, and hairy coverings 

 under these conditions. Plants that have a tendency toward 

 succulence become thicker and more succulent when subjected to 

 drought. The water-holding mucilages which they contain are 

 increased, and their water-holding capacity is enlarged. The 

 leaf area of mesophytes is greatly reduced by drought ; conse- 

 quently, photosynthesis and transpiration are also reduced. 

 Under the same conditions the mechanical tissues of woody plants 

 are increased, their stems become more rigid, and the bark be- 

 comes thicker and more impermeable to water. If we keep all 

 these facts in mind in studying the stems of xerophytes, we may 

 be able to understand better the causes of the pecuHar features of 

 this ecological group of plants. 



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