CHAPTER VII 



TRANSPORT OF WATER AND SOIL SOLUTES 



The Need of a Transporting System. — Unicellular plants 

 living in water or in moist and shady places can absorb water 

 and solutes throughout their entire surface, and in simple mul- 

 ticellular water plants consisting of a single row of cells or of 

 an expanse of tissue only one cell in thickness, each cell is in 

 position to absorb water and solutes for itself. But in bulkier 

 plants the interior cells have to draw upon the exterior for their 

 necessary materials. And where the distance to -the remoter 

 cells is great and loss of water through leaves and other above- 

 ground parts is considerable a system for the conduction of 

 water and solutes becomes imperative. 



A tissue made up of short cells will not serve this conduct- 

 ive purpose, as is shown by the fact that a strip of pith or 

 cortex with its lower end in water will soon begin to wither at 

 a height of 5 to 15 centimeters even when protected from rapid 

 loss of water; and experiments involving the extirpation of the 

 pith and the removal of the bark show that the water tubes in a 

 few thin vascular bundles can supply the water lost by trans- 

 piration through the leaves, while all of the tissues of the pith 

 and bark combined fail to do this. These things show us how 

 necessary has beeii the differentiation of a water-conducting 

 system as plants in their evolution have aspired to greater and 

 greater heights above the soil. 



Tissues Devoted to the Transport of Water. — The tracheal 

 tubes and tracheids have been shown to be the highways through 

 which water and solutes that have entered from the soil make 

 their way to the leaves. The origin and nature of these tissues 

 have already been told in Chapters II and III, and we have now to 

 consider their structural adaptations to the work they have to do. 



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