80 



A TEXTBOOK OF. BOTANY [Cn. Ill, 9 



FIG. 53. Transition from bud scales to leaf, w 4.u 



, . ,, , i ,. i . -M-, TMJ W1H1 



showing the former to be petioles, in Box Elder ; 



v i 

 *" 



the Australian Acacias, the chlorenchyma is all in the 

 petioles (called PHYLLODIA), which are vertically flattened 

 (Fig. 56), while the much compounded blades distinctive of 



Acacias are sup- 

 pressed. In other 

 cases the stipules 

 become enlarged, 

 aiding the blade in 

 its function as in 

 Violets (Fig. 57), 

 reaching to a size 

 and form identical 



p ft f f^p 



ofcJ Ul Lilc 



blades as in Gal- 



i 

 lum, or replacing 



the foliage altogether as in Lathyrus Aphaca (Fig. 51). The 



causes of these curious substitutions of functions are mostly 



not known, but they are presumably connected with pe- 



culiarities in the past history of the plants. For example, it 



seems likely that the abandon- 



ment of the leaf blade and 



transfer of the foliage func- 



tion to the petioles in Acacias 



represents a mode of adapta- 



tion to a climate increasing 



in dryness. Leaflets, which 



expose much horizontal sur- 



face, are out of place in 



dry climates, while a single 



petiole, flattened vertically, 



is better protected against 



extreme transpiration (page 

 70). 



One cannot but notice the diversity of form, and the 

 variety of apparent function, in the stipules. In existent 

 plants they seem to represent no distinctive organ, but 



FIG. 54. A cluster of spines from 



nooacto; < j. (After 



