540 cook: morphology and evolution of leaves 



differentiation of the ligule 



In the development of foliage leaves the ligule was probably 

 the first structure to be added after the sheath and the blade. 

 The ligule may be described as a thickened rim of the leaf sheath, 

 bearing the weight of the blade or holding the sheath in place 

 around the stem. In Serenoa and related genera of fan-palms 

 the ligule is supplemented by a ligule-like expansion of the rim 

 that supports the bases of the segments on the under side of the 

 leaf. Although these subligules, as they may be called, appear 

 in only a few palms, they are not without general interest as 

 organs that are closely parallel in structure and function with 

 the true ligules. 2 



The sheaths of most palms are thickened on the side that sup- 

 ports the petiole, and are split on the opposite side, allowing the 

 leaf to diverge more widely from the trunk, but the more primi- 

 tive condition of closed sheaths with nearly uniform texture is also 

 found, as in Calamus, Desmoncus, and Chamaedorea. Even in 

 the large royal palms and many others the sheaths are not split 

 until the leaves are ready to fall. 



In palms like Desmoncus and Pyrenoglyphis, as in many mem- 

 bers of the family Polygonaceae, the sheath, instead of being 

 specialized at the mouth to form a ligule, is prolonged far above 

 the insertion of the petiole, forming what is known as an ocrea. 

 A similar prolongation of the sheath beyond the point of attach- 

 ment with the petiole is found in the bud scales of Magnolia and 

 Ficus. Some writers have taken it for granted that the ocrea, 

 or the entire sheath, has been formed by the union of stipules, 

 but the indications point rather to the formation of stipules by 

 reduction and specialization of the sheaths of the more primi- 

 tive forms of leaves. 



Whether we consider that the blade arose simply as an expan- 

 sion of the upper portion of the sheath or as an outgrowth from 

 the rim of the sheath, the ligule may be considered as marking 



2 My attention has been called by Mr. H. Pittier to illustrations of the sub- 

 ligule of Trithrinax campestris published by C. De Candolle, Bull. Soc. Bot. 

 Geneve, II. 5: 106, pi. 3. 1913. 



