80 



A TEXTBOOK OF BOTANY [Ch. Ill, 9 



Fig. 53. — Transition from bud scales to leaf, 

 showing the former to be petioles, in Box Elder ; 



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 

 with those of the 

 blades as in Gal- 

 ium, 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 Goebei) 

 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 

 Echinocactus ; x J. (After 



