FLOWER-GARDENING. 131 



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

 upon, and whose existence is only to be discovered by acci- 

 dent. 



48. 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. If seems more probable that the oxygen of the atmo- 

 sphere, seizing upon a certain quantity of carbon, forms car- 

 bonic acid, which they absorb and feed 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 tenacious soils 

 are seldom suited to the purposes of the cultivator, until their 

 adhesiveness has been destroyed by the addition of other matter. 



52. It is therefore one of the most delicate parts of plants, 

 and the most easily injured. 



53. But to the other species the excrementitious matter is 

 either not unsuitable or not deleterious. 



54. Hence soil may be rendered impure (or, as we inaccu- 

 rately say, worn out) for one species, which will not be impure 

 for others. 



55. This is the true key of the theory of rotation of crops. 



56. This also may serve to explain in part why light soil is 

 indispensable to many plants, and heavy or tenacious soil suit- 

 able to so few ; for in the former case the spongioles will meet 

 with little resistance to their elongation, and will consequently 

 be continually leaving the place where their excrementitious 

 matter is deposited ; while in the latter case, the reverse will 

 occur. 



