Buds and Stems. 51 



are thin-walled, and the walls are made of cellulose, a sub- 

 stance which is easily permeable to water and substances 

 in solution. These cells are therefore fitted for the storage 

 and slow conduction of materials ; and since they lie near 

 the periphery and are accessible to light, they contain the 

 same kind of green chloroplasts which reside in the leaves, 

 and use the energy of the light in manufacturing food 

 materials (see page 87). The outer portion of the peri- 

 cycle, on account of the thickness and lignification of its 

 cell walls and of the elongation and close union of its 

 cells, is adapted to give strength and rigidity to the stem. 

 The inner portion of the pericycle, having thin, cellulose 

 walls, is adapted to the storage and slow conduction of 

 materials in solution. The phloem portion of the vascular 

 bundles probably carries proteid and other food materials 

 rapidly up and down the stem as needed, being fitted for 

 this purpose by its long, thin-walled, tubular cells, which 

 are separated longitudinally by partitions having perfora- 

 tions, through which materials may readily pass. The 

 xylem portion of the bundles carries water upward from 

 the roots through its tracheal tubes, while the wood fibers 

 associated with the tracheal tubes contribute to the strength 

 and hardness of the stem. When we consider the vascular 

 bundle as a whole we see that its conductive function is 

 preeminent. The medullary rays carry materials radially 

 to and from the bark and pith and to and from the vas- 

 cular bundles. The pith may, in its young state, conduct 

 materials up and down as needed, but as it gets older it 

 dies and contains air only, or it may break down entirely. 



Study also cross sections of an Aristolochia stem which is 

 several years old and note the changes which have taken 

 place in the different tissues since the first year. 



66. Make a diagrammatic drawing of the different zones 



