16 DISEASES OF TROPICAL PLANTS 



CH. 



in the formation of the food substances previously 



referred to. 



The STEM, as previously stated, is the part connecting 



the roots with the foliage, and through it must pass the 



water and the dissolved salts on their way from the 

 roots to the leaves. The elaborated food 

 substances which have been manufactured 

 in the leaves must also pass through the 

 stem to the other parts of the plant, 



FIG. 



ling plant show- 

 ing root hairs. 



FIG. 7. Particles 

 of soil held in place 

 by root hairs. 



FIG. 8. Root hairs. 



where they are to be utilised in the economy of the 

 growing plant. The stem, therefore, forms a connection 

 between the foliage and the roots, and also serves for 

 the conduction of both raw and elaborated food materials. 

 Its complicated branching arrangement facilitates the 

 bearing of an enormous amount of foliage, which is so 

 important in the economy of the growing plant. A 

 cross section of a dicotyledonous 

 stem (Fig. 9) shows it to be made 

 up of a number of fibro-vascular 

 bundles which are arranged in cylin- 

 drical form, surrounded by the 

 cortex on the outside and enclosing 

 the pith within. In young plants, 

 in the young parts of old plants, 

 and in soft succulent plants the 

 fibro- vascular bundles are separated 

 by masses of soft cells of the same 

 character as the cells in the pith. In old, woody plants 

 the bundles are dense and packed closely together. The 



FIG. 9. Cross section of 

 dicotyledonous stem. 



