ROOTS AND ROOT-HAIRS. 41 



It is a tubular prolongation of a single cell of 

 the external covering of the young root, usually 

 about I to 3 mm. in length, and 0.0 1 too. 10 mm. 

 in diameter. In special cases the root-hairs of 

 some water plants may reach 5 to 1 8 mm. in 

 length, but of course I am referring to the ordinary 

 land plants of agriculture and forestry. This 

 tubular prolongation is closed and rounded off at 

 the distal free end, and opens at the proximal end 

 into the cell of which it is a protrusion. 



The whole structure is bounded by an extremely 

 delicate and elastic wall of cellulose, which Frank 

 says is of special composition, almost too thin to 

 measure in many cases, but often somewhere near 

 0.005 to 0.00 1 mm. in thickness. This thin 

 membrane is remarkably permeable by water, or 

 dilute solutions, as is shown by the rapidity with 

 which a root-hair collapses if exposed to evapora- 

 tion, or with which dense solutions abstract water 

 from it, or with which solutions may be seen to 

 penetrate it under the microscope. 



Overlying the thin cell-wall proper, on the 

 outside, is a thin gelatinous layer, a product 

 of alteration of the outermost lamella; of the 

 former. 



Closely lining the proper cell-wall on the inside, 

 is an extremely thin layer of living protoplasm, 

 and somewhere in this protoplasm is a distinct 

 cell-nucleus. 



The interior of the tube is filled with cell-sap, 

 and it is the osmotic pressure of this cell-sap 

 which keeps the whole living instrument tense and 



