THE STEM 137 

which the strengthening material was arranged would, theoreti- 
cally, make comparatively little difference. The only stress 
would be a downward pressure, 
or compression, the resistance to 
which would depend more on the 
cross-sectional area than on the 
arrangement of the strengthening 
material. Actually, however, such 
a condition is seldom attained ; for 
the column, be it the stem of a 
plant or the support of a building, 
rarely remains permanently verti- - 
cal. When the stem of a plant be- 
fee? a compound girder com- comes inclined, as by the action of 
posed of I-beams, the webs of the wind or by the weight of the 
which have a common center branches, the side that 1s upper- 
most tends to become stretched, or 
develops tension, while the lower side is subjected to compres- 
sion. It therefore becomes important to have the strengthening 
material distributed near the upper 
and lower surfaces, or, in other words, 
in the form of an I-beam. As the 

Fic. 132. Diagram of cross sec- 
plant is likely to bend in any direc- 
tion, however, and thus may develop 
stress on any side, it is advantageous 
to have a number of these girders, 
with the webs crossing each other 
and the center of each at the center 
of the stem (Fig. 132). In the four- 
cornered stems of such plants as the 
mints or coleus the corners are occu- 
pied by a conspicuous development 
of collenchyma (Fig. 133), which 
thus forms the flanges of two I-beams, the webs of which are 
crossed. In many plants there are strands of sclerenchyma out- 
side of each vascular bundle (Fig. 134), and in such cases two 

Fig. 183. Diagram showing 
arrangement of vascular bun- 
dles and collenchyma (dotted 
area) in a four-cornered stem 

