THE LEAF. 



183 



leaves in Finns and Gingko ; the last-named may have 

 been derived from ancestors which were at once 

 reproductive and assimilating foliar organs. (The 

 relation between enations and ascidia and anther- 

 structure will be dealt with in Vol. II of this work.) 

 The progressive changes represent steps taken out of 

 the habitual course of the ancestry, and they must, 

 therefore, in most cases, be induced by special stimuli. 

 The changes are, in fact, in the direction of evolution, 

 and not the least value attaching to them may reside 

 in the indication which they possibly afford as to the 

 mode in which characters, varietal or specific, may 

 have arisen in the past. 



1. Squamody of Foliage-leaves. — Groebel observed 

 in Girema intermedia that if the overwintering stolons 

 are stimulated to further development in the winter 

 by cultivation in a higher temperature, " the point of 

 the shoot which ought to be an inflorescence becomes 

 a stolon, which again pierces the soil, and this may 

 take place after the shoot has attained a height of 

 many centimetres and formed a number of well- 

 developed leaves. The appearance of the shoots above 

 the ground also may be quite suppressed, and the 

 shoot, instead of forming a photophilous shoot with 

 foliage and flower, may continue its growth as a 

 stolon." In such cases those foliar rudiments on the 

 shoot which would under normal conditions develop 

 into foliage-leaves or floral organs, in the subterranean 

 habitat of the shoot develop as scale-leaves. 



Duchartre describes how in Lilium neilg/ierrense 

 some of the shoots emitted by the bulb, instead of 

 growing up vertically above-ground and forming 

 foliage-leaves, grow horizontally for a considerable 

 distance in the soil, producing scale-leaves only, 

 before finally turning upwards. 



2. Pampinody of Foi/iagf-leaves. — This occurs rarely, 

 as in Lathyrus sijivestrts, where the two lowest leaflets 

 were transformed into tendrils. And in the common 

 vetch (Vicia sativa) in which not only (as normally 



