THE LEAF. 



159 



other words, a xerophytic character. One has only 

 to visit such regions as the Cape Peninsula and the 

 Karroo, or a desert, to perceive that one of the nume- 

 rous modes of adaptation to the waterless condition is 

 that of a thorny leaf-structure. In our own country 

 we see the same phenomenon (which is more difficult 

 to account for, and seems to be better explained as a 

 protection from browsing animals) in the furze {JJlex 



Fig-. 42. — Cucurbita sp. a. Normal tendril, b. Transition from a 

 tendril to a foliage-leaf. (After Naudin.) 



euro])8eiis) where the entire leaf is transformed into a 

 thorn, and in the holly {Ilex aquifolium) where the leaf- 

 teeth are developed as such. Now it has been found 

 by experiment with certain plants, e. g. the furze and 

 thorn-acacia (Robinia), that, if grown in a rich soil and 

 in a very moist atmosphere, the thorns are entirely 

 replaced by trifoliolate leaves and the spiny stipules 

 disappear, which seems to indicate that poorness of 

 soil and dryness of air are at any rate factors in their 

 formation. In the barberry (Berberis) De Candolle 

 mentions transitional forms as abnormally occurring 

 between the foliage leaves of the short shoots and the 

 thorns of the long shoots. 



