THE LEAF. 



203 



two of its lobes, the result being that the quondam 

 terminal lobe becomes a dorsally-attached structure ; * 

 and we may probably account for its cup- or ascidium- 

 like form as follows. If the terminal lobe, shifted on 

 to the lower surface, retained its usual form, its upper 

 surface would be opposed to the lower surface of the 

 leaf bearing it, which would violate the law (which 

 appears to exist in spite of some apparent exceptions) 

 of laminar inversion ; hence the union of its basal 

 margins across the upper surface is necessary so as to 

 ensure its lower surface being opposed to that of the 

 rest of the leaf. The normal pitcher of Nepenthes has 

 probably originated in this way, but has become 

 secondarily shifted (in most species) into a purely 

 terminal position, i. e. back again into the original 

 position of the lobe from which the pitcher arose, the 

 latter having been, nevertheless, retained as a useful 

 structure in the economy of the plant. As a natural 

 conclusion from this view of the origin of the pitcher, 

 all that portion of the leaf below the pitcher cannot 

 be regarded as merely a winged petiole ; the history 

 of development certainly shows it to be the basal 

 portion of the whole leaf, but in the mature condition 

 it is indubitably the main assimilating part of the leaf; 

 moreover, we cannot allow the developmental history 

 to invariably afford data for morphological conclusions, 

 and no essential distinction can be recognized between 

 a basal and an upper portion of a leaf. 



Some of the abnormal leaves of Phlox described 

 above show us (PI. XXII, fig. 5) a very small basal 

 portion bearing the main part of the leaf as its dorsal 

 outgrowth. Normal instances of the same phenomenon 

 are seen in the leaf of the grass with its ligule formed 

 by lateral union of the stipular sheath-lobes across the 

 face of the leaf, in the coleoptile of the grass-embryo 

 and the axillary stipule of Potamogetoii and other plants, 

 in the glumes of Bromus and other grasses with their 



* Where a pitcher occurs on the upper surface of the leaf, the lateral lobes 

 have united across the lower face of the latter. 



