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 



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 a cen- 

 tral woody cylin- 

 der, they have an 



A B 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- 

 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 



FIG. 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 



FIG. 7. A cornstalk broken so as to show the num- 

 ber and distribution of the vascular bundles 



