50 COMPOUND ORGANS OF PLANTS. 



In exogenous stems of a single year's growth we therefore 

 observe, a central cellular pith, a zone of woody fibre and vas- 

 cular tissue, an exterior coating of bark, and medullary rays 

 passing from the pith to the bark. This is the complete struc- 

 ture of exogenous herbaceous stems which die down to the 

 ground annually. 



In exogenous stems which are not annual, at the close of 

 the growing season the stem ceases to elongate, the old leaves 

 gradually fall off, the new leaves, instead of expanding after 

 their formation, retain their rudimentary condition, harden and 

 fold over one another, and a bud is produced, the winter's 

 residence of the shoot. 



The second year's growth.-. — The next year, with the return of 

 light and heat to the earth in Spring, vegetation re-oommenoes. 

 The resinous exudation on the buds is melted by the heat of 

 the sun; the scales fall off, the leaves expand, and are sepa- 

 rated by the growth of the internodes, the buds terminal and 

 lateral are in this manner elongated into shoots, and are now 

 to the parent shoot what the young leaves were to it the first 

 year ; that is, they perform precisely the same functions, and 

 contribute by their downward growth, and their deposit of 

 woody and fibrous matter, to the increased diameter of the 

 parent shoot. 



With the development of the buds into shoots and leaves, 

 the sap is set into circulation through the system of the plant, 

 and the bark and wood whict, at the close of the growing 

 season, or in autumn, firmly adhered together, are now easily 

 separable from each other, by the formation between them of 

 a stratum of mucilaginous, organizable matter, termed cam- 

 bium. This cambium is nothing more than the ordinary sap, 

 the water of which having been evaporated in the leaves, is 



