176 



MECHANICAL F SYSTEM 



leading from the completely extra-cambial to the completely intra- 

 cambial form of mechanical cylinder. It is customary to discriminate 

 between the extreme types, and to restrict the term " bast-cylinder or 

 -ring " to the extra-cambial variety ; it has, however, already been 

 pointed out that this distinction rests upon a purely topographical 



Fio. 01. 



Mechanical cylinders of various Monocotyledons. A. Sub-cortical fibrous cylinder 

 of Allium odorum, with externally opposed mestome-bundles. B. Sub-cortical 

 fibrous cylinder of Convallaria verlicillata, with embedded mestome-bundles. C. Sub- 

 cortical fibrous cylinder, reinforced by sub-epidermal girders, of Molinia coerulea. 



basis. An interesting intermediate condition is found in the stem of 

 Astrantia major, where the leptome-portions of the vascular bundles are 

 embedded in the stereome-cylinder, whereas the hadrome-portions are 

 merely apposed to its inner face. The leptome and hadrome com- 

 ponents of the same bundle are thus separated by a strip of mechanical 



