CHAPTER VI 



THE ABSORPTION OF WATER AND MINERALS 



All substances that penetrate into the body of the plant cell 

 must be in solution, excepting in the case of low forms of plants 

 destitute of cell- walls which are sometimes' able to engulf solid 

 particles. Simple unicellular and filamentous algae can absorb 

 water throughout their entire surface, but more complex plants 

 from the liverworts and mosses upward to the seed plants, which 

 have ventured to raise a part of their bodies above the substratum 

 where the energy of the sunlight and materials of the atmosphere 

 can be more freely appropriated, have found it necessary to put 

 forth special absorbing organs into the substratum for the intake 

 of water and minerals; and the larger, taller, and more branched 

 the part above the substratum, the more extensive on the whole 

 must be the absorbing parts beneath the substratum. 



From the vascular cryptogams (ferns, lycopods, equisetums) 

 up through the seed plants, roots are employed for anchorage, 

 and for absorption and conduction to the stem, of water and 

 soil solutes. In floating water plants and in many aerophytes 

 (air plants) there are roots that serve for absorption and con- 

 duction only. 



Roots in the Soil. — It is only the younger parts of soil roots, 

 and particularly the root hairs growing near their apices, that 

 are fit to carry on absorption, the older parts having the walls 

 of the exterior cell more or less waterproofed. As will presently 

 be seen, the root hairs are the solution of some difficult problems 

 in the relation of plants to the soil. In order to penetrate the 

 soil in a necessarily sinuous course and to get past obstructions 

 in the best way the place of elongation in roots has been restricted 

 to the region of the apex, so that this delicate, sensitive part 

 might feel its way among the soil particles as it elongated, 



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