92 



ABSORPTION OF WATER AND MINERALS 



cording to recorded estimates. So that if the hairs were stripped 

 away the capacity for absorption would be correspondingly 

 reduced. The effect of this we see when young plants are trans- 

 planted, for even when great care is taken not to break the roots 

 wilting is apt to occur at first because the delicate root hairs have 



been torn off or have died away as a result 

 of the shock of transplantation. Soon, 

 however, new hairs are formed, water is 

 again absorbed in plenty, and the plant 

 picks up from its wilted condition. 



The protoplast of the mature root 

 hair is in the form of a very thin film 

 lining the wall, and the cavity which 

 it surrounds is filled with cell-sap con- 

 taining sugars, acids, etc., in solution 

 that afford osmotic conditions for the 

 intake of soil water with considerable 

 power (Fig. 44). 



Any substances from the soil, either 

 water or solutes, before mingling with 

 the sap in the root hairs, must pass 

 through the cellulose cell-wall, the ex- 

 ternal plasma membrane, the general 

 cytoplasm, and the internal plasma mem- 

 brane. The living plasmatic parts, how- 

 ever, taken altogether form a film so thin 

 as to be discerned with difficulty even 

 with high powers of the microscope. Not all substances in 

 solution in the soil water are able to make this passage. 

 Probably all of them can penetrate the cellulose wall, but to 

 some the external plasma membrane presents an impassable 

 barrier, and this membrane is then said to exercise a selective 

 function. Similarly the plasma membranes, external and 

 internal, keep the osmotic and nutrient substances of the cell- 

 sap from escaping. We must not, however, overlook the fact 

 that substances that have penetrated the wall of the root hair 



Fig. 44. — A single root 

 hair on a large scale, showing 

 that it is an outgrowth of 

 an epidermal cell, and the 

 fact that it possesses a living 

 protoplast and large vacuole 

 filled with cell-sap and 

 traversed by cytoplasmic 

 strands. The nucleus is 

 near the apex of the hair. 



