116 PLANT PHYSIOLOGY 



mation of the hairs, these must surely be dragged forward 

 and broken. Secondary growth in thickness of the part will 

 crush them. Each young and growing root or root branch 

 is covered for a time by a zone of root-hairs. This zone 

 will vary in breadth according to growth conditions. The 

 hairs will vary in length, diameter, and number according 

 to soil conditions. As the root grows, the work of the 

 older and less effective root-hairs is taken up by the younger 

 ones newly formed farther forward and nearer the growing 

 point. In this way new soil-particles are relieved of their 

 small stores of water, and the absorbing surface is cor- 

 related with the growth of the plant as weU as with its 

 demands in the stationary condition. 



THE MEANS OF TRANSFEB OF NUTRIENT SOLUTIONS 



Similar to the differences between cell-sap and soil-water 

 in density and in composition, which enable the root-hairs 

 to absorb water and dissolved food-materials, are the differ- 

 ences in the density and composition of the cell-sap of ad- 

 jacent cells. The cell with denser cell-sap will absorb water 

 from its neighbor with more dilute cell7sap, the cell-sap with 

 less of a needed food-material or food will absorb from one 

 with more. Such osmotic transfers are inevitable wherever 

 miscible solutions or liquids of different densities and compo- 

 sitions are on opposite sides of a permeable membrane and 

 in contact with it. By osmosis the distribution of food- 

 materials will take place through the body of a small land 

 plant, e, g. a fungus or a liverwort, and in submersed 

 aquatics. For all plants not subjected to the loss of water 

 by evaporation, and in the bodies of most land plants so 

 small and so simple as the liverworts and the fungi, the 

 rate of transfer by osmosis alone is rapid enough to ensure 

 the adequate distribution of water and of dissolved foods 

 and food-materials. Larger land plants are subjected to 

 such losses of water from their aerial parts that osmotic 

 transfer is too slow always to keep pace with evapora- 

 tion. In plants provided with special organs for excret- 

 ing water (see pp. 126-8) these organs must be copiously 

 supplied. 



