136 A TEXTBOOK OF GENERAL BOTANY 
least stress is in the center. For this reason it is usual to con- 
struct a beam in such a manner that it is more massive or com- 
posed of stronger material at the upper and lower surfaces than 
in the center. A usual form is the I-beam (Fig. 131). In this case 
the material is expanded at the upper and lower surfaces and 
connected by a much narrower portion. The extensions at the 
surfaces are called flanges. In bridge construction the connecting 
portion, known as the wed, is frequently made of latticework. 
Mechanical tissues in 
leaves. Ina leaf we have 
different conditions from 
those just discussed, in 
that the leaf is supported 
at only one end. The 
weight of the leaf gives it 
a tendency to bend down- 
ward so that its upper 
surface is stretched, or 
under tension, while its 
lower surface is under 
compression. As in the 
other case, the greatest 
stresses are at the upper and lower surfaces and the least stress 
is in the center. This arrangement, therefore, calls for longi- 
tudinal girders in which the strongest material is near the 
outer surfaces. The midrib and larger veins of the leaf repre- 
sent the girders. The principal strengthening material in these 
is usually collenchyma, and this is generally found as a broad 
band near the upper and lower surfaces just within the epi- 
dermis (Fig. 24). The collenchyma, therefore, represents the 
flanges of an I-beam, while the tissues between the two bands 
of collenchyma represent the web. The larger veins of grasses 
usually have sclerenchyma near the upper and lower surfaces, 
and so act as girders (Fig. 31). 
Mechanical tissues instems. If a column, such as a tree trunk, 
were supporting an evenly distributed weight, the manner in 

Pres eS | 4 

Fie. 1381. I-beam 
— se ee ee ee ee ae ee 
The wide portions at the top and bottom are the 
flanges ; the narrow connection is the web 
ee ee eee a — 
