THE MORPHOLOGICAL COMPOSITION OF PLANTS. 55 



represented in Fig. 80, will 

 be produced. 



When the successive fronds 

 are thus folded round so com- 

 pletely that their opposite 

 edges meet, these opposite 

 edges will be apt to unite : not 

 that they will grow together 

 after being formed, but that 

 they will develop in connexion; 79 



or, in botanical language, will become " adnate." That foliar 

 surfaces which, in their embryonic state, are in close contact, 

 often join into one, is a familiar fact. It is habitually so 

 with sepals or divisions of the calyx. In all campanulate 

 flowers it is so with petals. And in some tribes of plants 

 it is so with stamens. We are therefore well-warranted in 

 inferring, that under the conditions above described, the suc- 

 cessive fronds or leaflets will, by union of their remote edges, 

 first at their points of origin, and afterwards higher up, 

 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 a type (a being the mid-rib, and 

 b 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 tho 

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



