150 



ROOT. 



and mineral or vegetable poisons, will produce a much more 

 fatal effect upon the spongioles. 



37. These organs have'no power of selecting their food, but 

 will absorb wiiatever the earth or air may contain, which is 

 sufficiently fluid to pass through the sides of their tissue. 



38- So that if the spongioles are developed in a medium 

 which is of an unsuitable nature as they will still continue to 

 absorb, they cannot fail to introduce matter which will prove 

 either injurious or fatal to life, according to its intensity. 



39. This may often explain why trees suddenly become 

 unhealthy, without any external apparent cause. 



40. Plants have the power of replacing spongioles by the 

 formation of new ones ; so that an individual is not destroyed 

 by their loss. 



41. But this power depends upon the co-operation of the 

 atmosphere, and upon the special vital powers of the species. 



42 If the atmosphere is so humid as to hinder evaporation, 

 spongioles will have time to form anew ; but if the atmosphere 

 is dry, the loss by evaporation will be so much greater than 

 can he supplied by the injured roots, that the whole system 

 will be emptied of fluid before the new spongioles can form. 



43. This is the key to Transplantation. (XV.) 



44. As roots are destitute of leaf-buds (IV.), and as leaf- 

 buds are essential to the multiplication of an individual (108) 

 it should follow that roots can never be employed for the pur- 

 pose of multiplication. 



45. Nevertheless, roots when woody have, occasionally, the 

 power of generating adventitious leaY-buds (IV.) ; and when 

 this is the case, they may be employed for the purpose of 

 multiplication; as those of Cydonia Japonica, &c. 



46. The cause of this power existing in some species, and 

 not in others, is unknown. 



47. It is therefore a power that can never be calculated 

 upon ; and whose existence is only to be discovered by 

 accident. 



4b. Although roots are generated under-ground, and some- 

 times at considerable depths, yet access to a certain quantity 

 of atmospheric air appears indispensable to the healthy execu- 

 tion of their functions. This is constantly exemplified in 

 plants growing in the earth at the back of an ill-ventilated 

 forcing house, where the roots have no means of finding their 

 way into the earth on the outside of the house. 



49. It is supposed by some that the introduction of oxygen 

 into their system is as indispensable to them as to animals. 



50. It seems more probable that the oxygen of the atmos- 

 phere, seizing upon a certain quantity of carbon, forms carbonic 

 acid, which they absorb, and teed upon. 



51. It is at least certain that the exclusion of air from the 

 roots will always induce an unhealthy condition, or even 

 death itself. This may be one of the reasons why stiff tena- 



