CHAPTER V 

 THE PLANT SKELETON 



The cell-wall of unicellular plants such as Pleurococcus and 

 yeast is essentially an exoskeleton since it provides some degree 

 of strength and rigidity, and this is true of the cell-walls in 

 the higher plants; but where a plant on account of its size and 

 exposure to the elements is in danger of breaking down or 

 being crushed or torn it has been found necessary to set apart 

 certain tissues as a skeleton for the plant body as a whole, and 

 these tissues have been modified to become more effective as 

 skeletons and at the same time less efficient for other functions. 

 The need of a skeleton for the larger and more complex plants 

 is at once apparent. The larger the plant the greater is its ten- 

 dency to collapse on account of its 'own weight. Imagine a 

 tree trying to attain its normal size with all of its tissues like 

 elder pith or the pulp of an apple, or a toadstool presuming 

 to become the size of a tree without making any tissues like 

 bast or wood. The more branched a plant is the greater is its 

 danger of becoming dismembered, and the greater is the need 

 of the body being strong to support the branches, and of the 

 branches being firmly knit to the body. The more differen- 

 tiated the plant body becomes the greater is the danger attend- 

 ing dismemberment; and the greater also are the demands made 

 on the environment; and the catastrophe is correspondingly 

 more serious when any parts are torn away or thrown out of 

 their normal positions. Hence it is that the higher plants have 

 been under the necessity of building a skeleton of greater or 

 less strength and hardness. 



We find that for the purpose of a skeleton four tissues have 

 been wrought out, whose origins have already been told in 

 Chapter II, namely the collenchyma, the bast fiber tissue, the 

 wood fiber tissue, and the stone cell or stereid tissue. 



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