Ciiapter 15 



THE PLANT SKELETON 



It has already been emphasized that one of the basic differ- 

 ences between an animal and a plant lies in the nature of the 

 cell wall. The cell walls of plants, consisting largely of cellulose, 

 are hard and rigid; for this reason they can give a permanent 

 shape to the plant body in addition to serving as containers of the 

 protoplasm. Animals, on the other hand, because of the non- 

 cellulose character of their cell walls, if cell walls are present, 

 have the soft flesh typical of their kingdom and a body lacking 

 a firm shape unless special supporting tissues are present. Thus 

 all plants, due to the nature of their cell walls, possess a material 

 with some skeletal qualities distributed throughout their bodies. 

 But in addition, many plants have special supporting tissues 

 which function as skeletal elements. These usually involve the 

 addition of lignin instead of the calcium salts which form the 

 customary strengthening substances in the animal skeleton. 



Primitive multicellular plants such as the algae are pre- 

 dominantly found in aquatic habitats; thus the generalized 

 skeleton formed by the cellulose walls serves all their needs for 

 support. Kelps can grow to a length of several hundred feet even 

 though they lack special skeletal tissues, since their bulk is 

 partially supported by the buoyancy of the water. Special sup- 

 porting tissues are likewise lacking among such simple land plants 

 as are found in the Bryophyte phylum. Liverworts, being pros- 

 trate in habit, have little need for supporting tissues of any kind. 

 The more erect mosses, however, need stems with some degree of 

 rigidity. This is attained by the cellulose walls plus the turgidity 

 ,of the cells themselves. Turgidity, which accounts for the erect 

 lature of many plants which do not have adequately developed 

 :eletal elements, is the distension of the cell walls due to the 

 )ressure of the abundant water within the cell. Turgidity 

 plains the ability of many plants such as mushrooms or suc- 



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