THE MORPHOLOGICAL COMPOSITION OF PLANTS. 59 



form sheaths inserted one within another, and including the 

 axis. This incurving of the successive fronds, 



ending in the formation of sheaths, may be accompanied by 

 different sets of modifications. Supposing Fig. 81 to be a 

 transverse section of such type (a being the mid-rib, and 

 6 the expansion of an older frond ; while c is a younger frond 

 proliferously developed within it), there may begin two di- 

 vergent kinds of changes, leading to two contrasted struc- 

 tures. If, while frond continues to grow out of frond, the 

 series of united mid-ribs continues to be the channel of circu- 

 lation between the uppermost fronds and the roots if, as a 

 consequence, the compound mid-rib, or rudimentary axis, con- 

 tinues to increase in size laterally; there will arise the series 

 of transitional forms represented by the transverse sections 

 82, 83, 84, 85; ending in the production of a solid axis, 



everywhere wrapped round by the foliar surface of the 

 frond, as an outer layer or sheath. But if, on the other 

 hand, circumstances favour a form of plant which maintains 

 its uprightness at the smallest cost of substance if the 

 vascular bundles of each succeeding mid-rib, instead of re- 

 maining concentrated, become distributed all round the tube 



