MORPHOLOGY. 477 



known, those of the parts of the flower would follow as 

 a matter of course. 



It is not necessary, in this place, to pursue the sub- 

 ject of the development and construction of the leaf 

 further than they are illustrated by ordinary terato- 

 logical phenomena. 



From this point of view perhaps the most interesting 

 circumstance is the part that the sheath of the leaf 

 plays.^ In many cases of so-called metamorphosis, it is 

 the sheath of the leaf that is represented and not the 

 blade. In normal anatomy the sepals, petals, carpels, 

 and even the stamens, as a general rule, corre- 

 spond to the sheath rather than to the blade of the 

 leaf, as may be seen by the arrangement of the veins. 

 The blade of the leaf seems to be set apart for special 

 respiratory and absorbent offices, while the sheath is 

 in structure, if not in office, more akin to the stem. 

 It would not be easy apart fi'om their position to 

 distinguish between a tubular sheathing leaf and a 

 hoUow stem . The development of adventitious growths 

 by chorisis or enation has been frequently alluded to 

 in the foregoing pages, and many illustrations have 

 been given of the power that leaves have of branching 

 in more than one plane, owing to the projection of 

 secondary growing-points from the primary organ. 

 These new centres of development are closely con- 

 nected with the fibro -vascular system of the leaf, so 

 that no sooner does a new growing point originate, 

 than vessels are formed to connect the new growth 

 with the general fibrous cord, see pp. 355, 445. 

 This leads M. Casimir De CandoUe to consider the 

 entire leaf as a composite structure. The morpho- 

 logical unit, says he, is the cellular protrusion or 

 growing point (saillie) and its corresponding fibro- 

 vascular bundle.' 



The identity, in a morphological point of view, of 

 the leaves and the lateral parts of the flower is so 



See Clos., ' Bull. Soc. Bot. Fr.,' 1856, vol. iii, p. 679. 

 . ' Theorie de la Feuille,' p. 26. 



