STEM OF ZEA MAIS. 107 



be removed as far as possible towards the periphery. The 

 crowded peripheral vascular bundles, provided alike on the side 

 of the bast and of the wood with a strong cover of sclerenchyraa, 

 represent here a system of combined upright girders. The 

 sheaths of sclerenchyma are the ties ; the vascular bundles them- 

 selves are the "filling". The hypodermal cylinder of scleren- 

 chyma strengthens this action, even when, as in this case, not 

 very strongly developed. This hollow cylinder is mechanically 

 to be considered as a combination of numerous " ties," arranged 

 in a circle. 1 



It is desirable now to prepare radial longitudinal sections 

 through the stem. To obtain these take a piece of the stem 

 about I or inch long, cut it in two longitudinally through the 

 middle, and take the sections from the cut surface of either half.' 2 



1 I have felt unable to give a quite satisfactory translation of the above 

 passage in the text, and will therefore endeavour further to illustrate it. 

 Two sets of phenomena have to be mechanically provided for, the one affecting 

 the stem as a whole, the other its separate vascular bundles. First, as to the 

 stem as a whole. It has considerable weight to bear, its own and that .of its 

 leaves. It must therefore be mechanically constructed to resist crushing. It 

 has to bear often very great lateral strains, from winds. It must therefore 

 also be constructed to resist flexion. In both these respects it can be com- 

 pared with a pier of a bridge, especially a cylindrical iron pier of a lofty 

 railway bridge. To resist flexion this is made hollow, so as to throw all the 

 strength to the outside; and, to aid in resisting crushing, it may be filled 

 with concrete. Secondly, as to 1 the individual vascular bundles. The 

 sclerenchyma layers will help in the above purpose ; but the bundles, being on 

 the one hand water, and on the other hand food conduits, must be protected 

 mainly from the lateral strains which would tend to crush their elements, 

 make them " collapse," and so cease to function. This protection is the main 

 duty of the sheath of sclerenchyma. Its being most thickened on the inner 

 and outer side of the bundles, and taking thus the form largely of two arches 

 concave to each other, makes its structure the most advantageous for its pur- 

 pose, since all the lateral strains in such a cylindrical stem may be con- 

 sidered to be radial. To this we may add one more factor : the course of each 

 of the vascular bundles from its lower to its upper extremity is usually not 

 straight, but in the form of an elongated arrch, the concavity outwards, they 

 thus become akin to " struts '. If they anastomose, or join together, as they do 

 most beautifully in many cases, the mechanical analogy is still more com- 

 plete, since they then resemble the network of connecting girders or tie rods 

 with which all observers of iron bridges are familiar. I select iron bridges 

 for this illustration, for in them, as in nature, the smallest amount of mate- 

 rial is made to go the greatest possible way. [ED.] 



- An excellent method of preparing longitudinal sections of a stem, if 

 moderately thin, is to take a piece about | inch long, cut it nearly through 



