AERIAL STEMS . 147 



steins usually have more or less horizontal branches. In 

 the arrangement of its leaf-bearing branches the plant 

 appears to avoid as much as possible the shading of one 

 branch by another ; the branches are usually arranged so 

 that they secure for the leaves the maximum illumination, 

 that is, as much Ught as possible. 



The building of strong, erect stems is a problem in con- 

 struction which plants solve in two distinct ways : (i) Some 

 erect stems are hollow; the hard tissues are arranged in 

 the form of a hollow cyhnder; stalks of v/heat and the 

 fruit-bearing stems of dandehon- are good examples of 

 such stems. (2) More commonly, however, erect stems are 

 soUd ; the hard tissues are arranged in the form of a sohd 

 cylinder ; the trunk of a tree is an example of this arrange- 

 ment. Erect stems which are annual are either hollow, 

 or else their centers are occupied by that soft tissue called 

 pith; erect stems which are perennial are usually soHd. 



You have already noted that the tissue called xylem or 

 wood is composed of cells with thick walls. It is this 

 thickness of the walls of the xylem which principally gives 

 to stems their strength and stabiUty. The strength of the 

 xylem is frequently supplemented, however, by the strength 

 of what is called mechanical tissue. Mechanical tissue is 

 tissue which is of service to the plant simply by its presence ; 

 it does not participate directly in the Kfe processes of the 

 plant; its cells may be dead. Similarly, in your own 

 body, the hard parts of your bones are a sort of mechanical 

 tissue. Xylem, as it grows older, may become purely 

 mechanical tissue, as, for example, the older wood of 

 trees. 



The stems of trees are the most conspicuous of erect 

 stems ; the most conspicuous, in fact, of all aerial stems. 



