164 



PRINCIPLES OF PLANT-TERATOLOGY. 



They occur in the oak (Quercus Robur var. filicina), 

 birch (Betula alba var. dalecarlica), alder (Ahius gluti- 

 nosa var. laciniata), hazel (Gorylus Avettana var. laci- 

 niata), laburnum (Gytisus Laburnum var. q iter ci folium), 

 beech (Fagus sylvaticaYav. heterbpkylla), and oak-leaved 

 foliage of the honeysuckle (Lonicera Periclymenum) 

 which has grown in shady woodland. 



All these cases are probably due to depauperization, 

 or reduction in the available assimilating surface of the 

 leaf ; and may be, as almost certainly in the case of 

 the honeysuckle mentioned, and possibly also in that 

 of the forest-trees, due to absence of sufficient sun- 

 light. On the other hand, it is possibly a reversionary 

 phenomenon, for in some of the genera concerned, 

 incised or lobed leaves are a normal feature of certain 

 species, e. g. in the Turkey oak (Q. Gerris), Japanese 

 honeysuckle (L. japonic a), and snowberry (Symphori- 

 carpus racemosus) . 



Again, other kinds of heterophylly seem to be 

 rather attributable to hypertrophy, i. e. to an effort 

 on the part of the leaf to increase its area and to 

 reproduce itself. Possibly some of the above-cited 

 cases of laciniation may come under this heading. 

 The most remarkable instance is afforded by certain 

 varieties of the common polypody (Polyp odium vul- 

 gar e), e.g. the variety cornubiense in which the usually 

 entire and simple lateral lobes elongate and themselves 

 become pinnately divided, giving the whole frond a 

 beautiful and elegant appearance ; either all the lobes 

 of the frond or only some of them may be affected in 

 this way. The hart's-tongue (Scolopendrium vulgare) 

 is extremely subject to marginal incision of the normally 

 quite simple and entire frond, taking the form of 

 crisping (var. endiviaefolium), erosion (erosum), lobing, 

 or pinnatisection. There are certain fixed and orna- 

 mental varieties of the cabbage (Brass ica oleracea) 

 known as borecole or kale in which the leaves are 

 crisped at the margins owing to their being abundantly, 

 though not very deeply, incised. The same pheno- 



