OF THE STEM OR ASCENDING AXIS. 733 



place unless the roots of such plants were injured. While it 

 may be proved, therefore, that excretions from the roots can 

 have no influence on the rotation of crops, still it is by no means 

 proved that roots do not impart some of their contents to the 

 soil. The evidence, so far as we are able to judge, would seem 

 to lead to the conclusion, that roots have no power of getting 

 rid of excrementitious matters like that possessed by animals, 

 but nevertheless that they do throw off into the soil a portion of 

 their contents by a process of exosmose, which appears to be 

 a necessary result and accompaniment of endosmose by which 

 absorption takes place. 



Roots are frequently enlarged for the purpose of acting as 

 reservoirs of nutriment in the form of starchy, gummy, and 

 similar matters, for the future support of the plant. The tuber- 

 cles of the dahlia {fig. 244) and orchis {figs. 242 and 243), and 

 the roots of the turnip {fig. 250) and carrot {fig. 248), are 

 familiar illustrations of such roots. 



2. Of the Stem or Ascending Axis. — The oflfices per- 

 formed by the stem and its ramifications are: — 1. To form a 

 support for the leaves and other appendages of the axis which 

 have but a temporary existence, and thus enable them to be 

 freely exposed to the influences of hght and atmospheric air, 

 which are essential for the proper performance of their functions 

 and development ; 2. To convey air and fluid matters upwards, 

 downwards, and inwards, to the organs of respiration, assimila- 

 tion, development, and secretion; and 3. To act as a reservoir 

 for the secretions of the plant. 



Special Functions of the different Parts of the Stem. — 1. The 

 Pith. — Various functions have at different times been ascribed 

 to the pith. In the young plant, and in all newly formed parts, 

 the cells of the pith are filled with a greenish fluid containing 

 gum and other nutritious substances in a state of solution. As 

 the parts increase in age the pith loses its colour, becomes dry, 

 and is generally more or less destroyed. Hence it appears to 

 serve the temporary purpose of nourishing the parts which sur- 

 round it when they are in a young state. The pith also, in 

 some cases, acts as a reservoir for the secretions of the plant. 



2. The Wood. — The wood-cells, when in a young and per- 

 vious condition, are the main agents by which the crude sap is 

 conveyed to the external organs to be aerated and elaborated ; 

 whilst the vessels of the medullary sheath and other parts are 

 carriers of air through the plant, and only in exceptionable 

 cases convey fluids. (See pages 726 and 727.) As the wood 

 increases in age, the tissues of which it is composed become 

 filled with various secretions by which they are hardened and 

 solidified, and in this manner the stem acquires strength ^nd 

 firmness. On the outside of the young wood, but organically 



