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A TEXTBOOK OF THEORETICAL BOTANY 



notably the Umbelliferae. In many jVIonocotyledons the vegetative stem 

 remains very short, and a large part of what appears to be the leafy axis is 

 reallv a cylinder formed by the tightly wrapped leaf sheaths of successive 



Fig. 980. — Polamo'-etou perfoUatus. Intravaginal 

 scales. {After Aschersou.) 



leaves, up through the middle of which eventually grows the flowering axis. 

 This structure is familiar in many Grasses and Sedges but reaches its greatest 

 development in the Banana, where it attains tree-like proportions. 



Eichler distinguished leaf sheaths which were composed of stipules more 

 or less adnate to the leaf base from those in which the sheath was a simple 

 expansion of the petiole or leaf base itself. Most leaf sheaths in Dicotyledons 

 are of the former nature ; only in a few families, such as the Compositae. 

 where no stipules exist, is the leaf sheath indubitably petiolar. The Mono- 

 cotyledons rarely possess free stipules, but in them the leaf sheath, which is 

 almost universal, is probably also the product of fused stipules, united to 

 the leaf base. In both groups of Angiosperms the tips of fused stipules may 

 remain free, appearing as a scale at the top of the sheath, known as the ligule, 

 which often shows, by its division into two lobes, an indication of its double 

 nature. Indeed, it has been pointed out that ligule and sheath bear an inverse 

 relationship to each other and that the shorter the sheath is, the larger ?nd 

 more stipule-like is the ligule {e.g., in Trifoliiim). 



Thirdly, when the laminar portion of a leaf rudiment is suppressed 

 during its development, and the resulting mature structure assumes the 

 appearance and assimilatory function of a leaf, although actually incomplete, 

 it is called a phyllode. True phyllodes are not as numerous as they may 

 appear to be on a superficial survey of Angiosperms. The name has been 

 freely applied to cases where the developmental history is unknown and 



