100 



PRACTICAL COURSE IN BOTANY 



plants of this class, with a few exceptions, like smilax and 

 asparagus, are characterized by simple, columnar stems and 

 a limited spread of leaves. Such plant forms are admirably 

 adapted by their structure to the purposes of mechanical 

 support. It is a well-known law of mechanics that a hollow 

 cylinder is a great deal stronger than the same mass would 

 be in solid form, as may easily be tested by the simple ex- 

 periment of breaking in your fingers a cedar pencil and a , 

 joint of cane or a stem of smilax of the same weight. In 

 stems that may be technically classed as solid in structure, 

 like the corn and palmetto, the interior is so light compared 

 with the hard epidermis that the result is practically a hollow 

 cylinder. 



114. Minute study of a monocotyl stem. -- Place under 

 the microscope a very thin transverse section of a cornstalk. 



The little dots that looked like 

 the cut ends of threads to the 

 naked eye will now appear as 



. 115. Transverse section through^ 



the fibrovascular bundle of a cornstalk r^HFia^lliLr Vertical section of the same ; 

 a, annular tracheid ; sp, spiral tracheid ; a and a', rings of a decomposed annular 

 m and m', ducts ; I, air space ; v, sieve tracheid ; v, sieve tubes ; s, companion 

 tubes ; s, companion cells ; vg, strength- cells ; cp, bast ; I, air space ; vg, strength- 

 ening fibers ; cp, bast ; /, /, parenchyma, ening tissue ; sp, spiral duct. 



the complex group of cells shown in Fig. 115. The same parts 



a.rp shown Inncnt.nrlirmllv in TTio- lift As sppn in nrnss spr- 



