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PLANT PHYSIOLOGY AND ECOLOGY 



178. Storage leaves are distinguished by the water storage cells, 

 or tissue developed in the chlorenchym. They usually show a well- 

 developed cuticle, with several rows of palisade tissue, and may 

 consequently be regarded as a special modification of the cutinized 

 leaf. The storage cells maintain a reserve supply of water, which 



Fig. 50. Cross-section of the two types of storage leaves. The storage 

 layers are transverse in Mertensia linearis, and vertical in Grindelia 

 squarrosa. 



is slowly yielded to the other cells in time of extreme drouth. 

 They differ from the cells of palisade or sponge in size and shape, 

 but their origin from these is indicated by the fact that in some 

 species chloroplasts are still present, though reduced in number. 

 Water cells usually occur in plates or layers, which may be at 

 right angles to the surface of the leaf or parallel with it. Xero- 

 phytic species of Mertensia, Erigeron, etc., illustrate the more fre- 



