THE LEAF. 



201 



extended nearly the whole length of the midrib of the 

 lower surface and represented an entire leaf, but of 

 much smaller size than that of its parent. 



The same plants of Phlox paniculata afflicted with 

 eel worm which bore the dorsal outgrowths from the 

 leaves also produced leaves which bore laminar out- 

 growths, to the number of one, two, and four, according 

 to the leaf, on the upper surface ; these extended some 

 distance along the veins (PI. XXII, fig. 5) ; they fre- 

 quently have little pointed apices of their own, they 

 also, in some leaves, form a pair of basal pockets, as in 

 Polygonum. In Saxifraga ligulata, besides pitcher- 

 formation on the lower (dorsal) surface, the same type 

 of enation as just described in Pit! ox and Polygonum, 

 occurs on the upper surface : the midrib, along its 

 whole length, may exhibit on either side a crumpled 

 wavy outgrowth accompanied often by formation of a 

 pair of basal pockets (PI. XVIII, figs. 2 and 3). In a 

 later chapter is described the occurrence, on the normal 

 foliage of various species of Caltha, of enations on the 

 upper surface. 



In the case of a vine the attachment was by the 

 upper surfaces of the petioles only ; it was unknown 

 what position on the stem the double leaf so formed 

 occupied. 



Conclusions on Simplification. — Having regard to 

 the fact that the well-developed foliaceous sporophyll 

 would seem to be the ancestor (so far as our knowledge 

 of plant-evolution at present tends to show) of all other 

 types of foliar organ, we* must envisage most of these 

 cases of simplification as, not reversionary, but pro- 

 gressive phenomena, i. e. as modifications of an organ 

 which, before it changed, was much nearer the original 

 type. Few w^ill deny that a tendril, a thorn, and a scale- 

 leaf are modified foliage-leaves; therefore, when we see 

 a foliage-leaf or leaflet being replaced by any of these 

 organs the change must be regarded as a forward and 

 not a backward one. 



