STEM. 123 



50. It seems more probable that the oxygen of the atmosphere, seizing 

 upon a certain quantity of carbon, forms carbonic 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 bo 

 one of the reasons why stiff tenacious soils are seldom suited to the pur- 

 poses of the cultivator, until their adhesiveness has been destroyed by the 

 addition of other matter. 



52. Spongioles secrete excrementitious matter, which is unsuitable to the 

 same species afterward as food ; for poisonous substances are as fatal to the 

 species that secrete them as to any other species. 



53. But to the other species the excrementitious matter is either not un 

 suitable, or not deleterious. 



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

 ble to many plants, and heavy or tenacious soil suitable to so few ; for in 

 the former case the spongioles will meet with little resistance to their elong- 

 ation, and will consequently be continually leaving the place where theii 

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

 will occur. 



III. Stem. 



57. The Stem is that part of a plant which is developed above ground, 

 and which took an upward direction at the period of germination. 



58. It consists of a woody axis, covered by bark having stomata (131) 

 on its surface, bearing leaves with leaf-buds, in their axillae, and producing 

 flowers and fruit. 



59. The points where leaves are borne are called Nodi; the spaces be- 

 tween the leaves, are Internodia. 



60. The more erect a stem grows, the more vigorous it is ; and the more 

 it deviates from this direction to a horizontal or pendulous position, the 

 less is it vigorous. 



61. Some stems are developed under ground, such as the Tubers of the 

 Potato and the Cormus of the Crocus ; but they are known from roots by 

 the presence of leaves, and regular leaf-buds, upon their surface. 



62. Stems increase in diameter in two ways. 



63. Either by the addition of new matter to the outside of the wood 

 and the inside of the bark ; when they are Exogenous / ex. Oak. 



64. Or by the addition of new matter to their inside ; when they are 

 Endogenous ,- ex. Cane. 



65. In Exogenous stems, the central portion, which is harder and darker 

 than that at the circumference, is called Heart Wood; while the exterior, 

 which is softer and lighter, is called Alburnum, or Sap Wood. 



66. The inside of the bark of such stems has also the technical name of 

 Liber. 



67. The heart wood was, when young, Alburnum, and afterward 

 changed its nature by becoming the receptacle of certain secretions peculiar 

 to the species. 



