THE PLANT AS A WORKING MACHINE 11 
9. Stems as supporting structures. In an examination of 
the stem of almost any woody plant there appears an outer dead 
bark, an inner green bark, a cylinder of wood, and sometimes a 
Fie. 6. Root hairs 
A, a cross section of a root, showing the central cylinder 
(cyl), the cortex (co), and the epidermis (ep), with root 
hairs that have grown from epidermal cells. B, a root 
hair enlarged to show the soil particles attached to it 
small area of pith 
in the center of 
the woody cylin- 
der. Although 
some of our com- 
monest agricul- 
tural plants, as 
corn and wheat, 
do not have acen- 
tral woody cylin- 
der, they have an 
outer region of 
very hard, strong 
tissue, and either 
a hollow central 
region, as in the 
wheat, or a large 
pithy center through which run a number of fibrous bundles, as 
in corn (fig. 7). In wheat the fibrous bundles lie close to the hard 
outer portion of 
the stem; conse- 
quently they are 
not seen nearly 
so easily as are 
the fibrous bun- 
dles in corn. A 
stem could not 
stand alone in an 
upright position 
if it did not pos- 
A cornstalk broken so as to show the num- 
ber and distribution of the vascular bundles 
sess some such rigid tissues as those generally noted in woody 
plants or in the outer portions of corn and wheat stems. Other 
factors that help in maintaining an upright position of the 
