LEAVES 



641 



In some instances petioles are of value in protecting developing buds from trans- 

 piration and other detrimental factors, as in Acer, Platanus, and Rhus, where 

 the buds for the next season are hidden more or less completely under the base of 

 the petiole until leaf fall, by which time the bud scales are fully formed (figs. 940, 

 941). In various developing umbellifer shoots (as in Angelica, fig. 942) the petioles 

 have large sheathing bases which enclose all the younger parts. In some aquatic 

 plants the petioles are greatly inflated, air-containing organs that help to float the 

 inflorescence (as in Utricularia inflata) or the entire plant (as in Eichhornia). 

 Variations in the length of petioles, due to external factors, will be considered 

 under stems (p. 728). 



Stipules. — Many plants, especially dicotyls, possess leaf appendages known as 

 stipules, which usually occur in pairs, one at each side of the petiole near its base. 



Figs. 942-944. — 942, a growing shoot oi Angelica airopurpurea, showing the large inflated 

 petiole (p) of a full-grown leaf, from which a young leaf (/) is just emerging; the petiole 

 (J>') of this young leaf is cut away enough to show within it a still younger leaf (/') ; the 

 petiole (/>") of this younger leaf contains within it a still smaller and younger leaf ; 943, a 

 part of a young shoot of a wild pea (Lathyrus ochroleucus), showing the prominent stipules 

 {s, s'), and a young leaf (/) terminated by a tendril (/); the stipules early develop to 

 their full size, for a time being erect and enclosing an undeveloped shoot, as at s' ; 944, a 

 growing shoot of a loosestrife (Lysimachia), showing a gradual transition from the early 

 scale leaves {s) to the foliage leaves (/) that appear later; note also the decussate 

 phyllotaxy. 



Their presence or absence apparently is unrelated to external factors and often charac- 

 terizes entire genera or families ; the Rosaceae and the Leguminosae, for example, com- 

 monly have stipules, while the Cruciferae and the Ranunculaceae commonly have 

 none. Stipules assume different forms, appearing as spines in Robinia, and is mem- 



