OF THE ROOT OR DESCENDING AXIS. 731 



able circumstances of a warm soil and moist atmosphere the 

 destruction of a large portion of the young extremities of the 

 root will do but httle injury, as the plant will then speedily form 

 new absorbent extremities, but if the conditions of the earth 

 and soil be the reverse of those stated above, then a large de- 

 struction of the young extremities of the roots will cause the 

 plant to die before new absorbent extremities can be formed. 

 Special attention should be paid to the above facts when trans- 

 planting is performed in the growing season; but it is far better, 

 when possible, to transplant late in the summer or in the autumn 

 when the growing season is drawing to a close, or in the spring 

 before it has recommenced, as at such periods, httle or no ab- 

 sorption takes place, and the plants have accordingly time to 

 recover themselves, before they are required to perform any 

 active functions. (See pages 779 and 780.) 



This absorption of food by the young extremities of the roots 

 is due to endosmosc taking place between the contents of their 

 cells and the fluids of the surrounding soil, and the immediate 

 cause of this action is the greater density of the cell-contents 

 of the roots produced by the vital actions which are there going 

 on. (See page 781.) 



Roots, as we have already seen (page 119), only grow in 

 length by additions near to their extremities, and as it is at 

 these parts that absorption of food almost entirely takes place, 

 they are always placed in the most favourable circumstances for 

 obtaining it, because in their growth they are constantly entering 

 new soil, and hence, as one portion of that soil has its nutritious 

 matters extracted, another is entered which is in an unexhausted 

 state. 



Roots can only absorb substances in a liquid state, and hence 

 the different inorganic substances, &c. which are derived from 

 the soil, and which form an essential part of the food of plants 

 (page 777), must previously be dissolved in water. If the roots 

 of a freely growing plant be placed in water in which charcoal 

 in the most minute state of division, has been put, as that sub- 

 stance is insoluble in the fluid, it will remain on the surface of 

 the roots, and the water alone will pass into them ; thus showing 

 that substances in solution are alone taken up by roots. 



Various experiments have been devised to ascertain whether 

 the plant possesses any power of selecting food by its roots. 

 Saussure proved, that when the roots of plants were put into 

 mixed solutions of various salts, some were taken up more freely 

 than others. He also found, that dead or diseased roots absorbed 

 differently to those in a vital condition. The experiments of 

 Daubeny, Trinchinetti, and others, lead essentially to the same 

 conclusions. Again, if the seeds of two different plants, such 

 as the common bean and wheat, be sown in the same soil, and 

 exposed, as far as possible, to the same influences in their after 



