LEAVES 



599 



Not only may small, thick leaves be developed where the air and the 

 soil are dry, but also in the presence of any factor that impairs root 

 absorption. For example, it has been shown in the case of more than 

 two hundred species that portions of the same individual plant develop 

 more xerophytic leaves when grown in alpine regions than when grown 

 in the lowlands (figs. 869, 870; also figs. 1051, 1052); this result doubt- 

 less is due in part to 

 reduced absorption on 

 account of low tempera- 

 ture and perhaps in part 

 to increased transpira- 

 tion. Similar results have 

 been obtained in maritime 

 cultures, absorption being 

 impaired by the presence 

 of sodium chlorid, and in 

 bog cultures, where the 

 fact of impaired absorp- 

 tion is variously explained 

 (p. 537); the small, thick 

 leaves of a bog individual 

 of Ledum groenlandicum 

 contrast strikingly with 

 the large, thin leaves of 

 a forest individual; the 

 maritime forms of many 

 species have thicker leaves 

 than do the inland forms, 

 chiefly by reason of 

 greater palisade develop- 

 ment.' 



Figs. 869, 870. — Experimental variation in the 

 dandelion {Taraxacum officinale): 869, a plant grown 

 in a lowland garden, having large and relatively thin 

 leaves and long scapes (s)\ 870, a plant grown in an 

 alpine garden (originally a portion of the same in- 

 dividual as 869), having small and relatively thick 

 leaves and short scapes (s') ; note that the root and the 

 inflorescence are much less reduced than are the other 

 organs; both figures are drawn to the same scale. 

 — From Bonnier. 



The theory above outlined 

 may be called the water theory 



of leaf size and proportion, since the absorption and the evaporation of water appear to 

 be controlling factors. There is another prominent theory, however, in which light is re- 

 garded as the dominant factor. Adherents of the light theory speak commonly of sun 



' It may be worth noting that submersed leaves are as thin and as expanded in bog 

 ponds as in other ponds, in spite of the supposed presence of substances unfavorable to 

 absorption; however, the absence of transpiration in such plants may be the significant 

 factor. 



