88 PLANT-LIFE ON LAND [CH. 



that the absorption of water and of the salts dissolved 

 in it is localised. These are abstracted from the soil 

 by the roots, to be carried by the transpiration stream 

 to the foliage, where the water evaporates away, 

 leaving the salts in the very region where the con- 

 struction of new organic stuffs is proceeding. More- 

 over the supply of food stuifs is further facilitated by 

 constant movement of water in the soil. This, together 

 with the growth and spreading of the roots, and the 

 recurrent renewal of their absorbing surfaces, result 

 in conditions which bring the plant in touch with ever 

 fresh supplies of raw materials. It is thus seen that 

 the fixed position of the plant in the soil is a necessary 

 condition of successful nutrition in all the ordinary 

 vegetation of the land. A measure of its effectiveness 

 may be seen in the permanence of large trees, such 

 as the Sequoias, or Mammoth Trees of California, 

 which bear evidence of individual growth for over 

 1000 years. 



The facts alluded to in the preceding paragraphs 

 are so familiar that one is apt to neglect the important 

 restrictions which necessarily follow from them. In 

 considering these it will be well to compare the 

 higher plants, which are typically fixed to the sub- 

 stratum, with the higher animals which are ambulatory 

 in the habit. So we shall see actually worked out in 

 practice the consequences which this difference in 

 habit has entailed. The most important disabilities 



