STEM. 151 



clous soils arc- so seldom suited to the purposes of the cultiva- 

 tor, until their adhesiveness has heen destroyed by the addition 

 of other matter. 



52. Spongioles secrete excrementitious matter, which is 

 unsuitable to the same species afterwards as food ; for poison- 

 ous substances are as fatal to the species that secrete them as 

 to anv other species. 



53." But to other species the excrementitious matter is cither 

 not unsuitable, or not deleterious. 



64. II* nee, soil may be rendered impure (or, as we inaccu- 

 rately say, worn out) fo* 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 so many plants, and heavy or tenacious 

 soil suitable to so few : for in the former case the spongioses 

 will meet with little resistance to their elongation, and will 

 consequently be continually quitting the place where their 

 excrementitious matter is deposited ; while in the latter case, 

 the reverse will occur. 



111. Stem. 



97. The stem is that part of a plant which is developed 

 above ground, and which took an upward direction at the 

 period of germination- 



53 It consists of a woody axis, covered by bark having 

 stomata (131. )on its surface, bearing leaves with leaf-buds, in 

 their axilla, and producing flowers and fruit, 



59 The points where leaves are borne are called Nodi; the 

 spaces between the leaves are lutanodia. 



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 underground, 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. 



G2. 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 j when 

 they are Endogenous ; ex. Cane. 



65. In Exogenous stems, the central portion, which is 

 harder and darker than tnat at the circumference, is called 

 Heart-wood; while the exterior, which is softer and lighter, is 

 called Alburnum or Sap-uood. 



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

 technical name of Liber. 



