3O Plants and their Ways in South Africa 



Of the great root system of trees, only the tips are absorbing 

 food material. As they push forward, the tips are protected by 

 a short-lived cap of tissue, which is constantly being renewed 

 as it is worn off. 



Water and solutes (mineral substances dissolved in water) 

 enter the root by osmosis * and diffusion, and pass into the 

 bundles of long slender tubes (vascular bundles), that continue 



from the root through 

 the stem to the very 

 tips of the leaves. 'I he 

 tubes are not continuous 

 throughout the length 

 of the plant but the con- 

 tents pass from one tube 

 to another adjacent one. 

 Water is required for 

 food material, some is 

 retained within the cells, 

 the cell walls are per- 

 meated with it and part 

 of the water taken in 

 passes out through the 

 leaves and stem into the- 

 air. The escape of 

 water is called tran- 

 spiration. 



Cell sap contains sugar and acids. These cause the inflow 

 of water by osmosis. The water constantly passes from cells 

 with less dense to those with more dense osmotic substances. 

 If transpiration hastens the upward ascent of water the solutes 

 are not necessarily hastened since they obey different condi- 

 tions.- Solutes pass from cells having a greater to those having 

 a less concentration of solutes. 



The water, in its long journey, must not be dried up, and 

 to guard against this, under the thin outer dress of stems and 



1 See p. 57. 



2 In a tobacco field Hasselbring (" Bot. Gaz." Jan. 1914) found that 

 plants which absorbed and transpired the most water contained a smaller 

 percentage and quantity of ash. 



FIG. 36. G, central cylinder, consisting of 

 pith, vascular bundles, and pericycle ; ft, 

 cortex ; A', piliferous layer ; W, root-cap. 

 (From Edmonds and Marloth's " Elemen- 

 tary Botany for South Africa ".) 



