34 The Nature and Origin of Stipules. 



have developed into a minute blade and petiole. The third leaf is 

 of the adult form, but smaller, though all the parts have increased 

 very much in size. This is contrary to what is observed in 

 Ranunculus where the sheathing petiole degenerates while the 

 other parts advance. The margins of the sheathing petiole of 

 Richardia curve inward at their apices and meet in the middle 

 line of the leaf as in the case of Thalictrum polygamum L., but 

 they are much broader and form a distinct ligule which is sup- 

 ported by the incurving and union of the marginal veins of the 

 sheath instead of by tangential branches. In Arissema triphyl- 

 lum (L.) Torr., the transition is not so well marked, owing to the 

 small number of leaves the first of which is but a sheath as in 

 Richardia, while the second bears a mature lamina. 



Scirpus polyphyllus Vahl. (fig. 2 7) will serve well to illustrate 

 the ligule in the Cyperaceae. It is but little developed as a slight 

 hyaline outgrowth upon the ridge at the union of the sheath and 

 lamina, but the sheath is closed, as is typical in the family, and a 

 little farther development of marginal tissue would produce an 

 ochrea. Typical of the ligule in our common grasses is that of 

 Phalaris arundinacea L. (fig. 28). It consists of a considerable 

 outgrowth of hyaline tissue which is continuous laterally with the 

 marginal hyaline tissue of the sheath. This continuity strongly 

 supports the position taken as to the origin of the ligule. The 

 purpose of the ligule is evidently to prevent the flow of water 

 from the upper parts of the leaf down between the sheathing pet- 

 iole and the stem which together with the axillary bud it invests 

 and protects, and neither the ligule nor the primitive ridge which 

 bears it are found in those cases where the sheathing petiole does 

 not closely invest the stem, at least in the early stages of growth, 

 and its purpose could not be in any considerable measure ful- 

 filled. 



The usually axillary position of the " stipule " in the Naiada- 

 ceae has occasioned considerable discussion as to its real re- 

 lation to the ligule of grasses and to stipules proper. That it 

 is in reality a development of the lateral portions of the primi- 

 tive leaf, and that it corresponds to the ligule together with 

 the margins of the sheathing petiole of grasses and is rendered 

 more or less nearly axillary by the degeneration of the central- 

 basal portion, becomes clear from the fact that in some species of 

 Naiadaceae the sheathing petiole retains a considerable degree of 



