The Nature and Origin of Stipules. 33 



portions, hyaline in texture and requiring no special support other 

 than that of the surrounding leaves, are rounded off distinctly at 

 the top at the point of beginning of the true petiole. In the 

 fifteenth leaf (fig. 23) there is a further reduction in size and the 

 tips of the lateral portions are free. Another interesting case 

 among the Ranunculacea? is that of Thalictrum pohjrjavium Muhl. 

 in which the sheathing petiole is of a very generalized type (fig. 

 24). The lateral portions are chiefly hyaline, though sometimes 

 faintly netted-veined and their margins turn in at the apex and 

 meet in the central dorsal channel of the petiole at its base, form- 

 ing a ridge between the sheathing and true petioles. This ridge 

 supports a very narrow hyaline membrane which appears to me 

 as the rudiment of a ligule. It would become typical b3^ a little 

 further development of marginal tissue. I believe this to be the 

 origin of the ligule wherever it occurs, though it does not appear 

 so clearly evident in highly specialized groups, nor should we 

 expect such to be the case. There is also present at the first and 

 second forkings of the petiole a transverse h^-aline scale very 

 much like a ligule. 



It is noteworthy that the ligule always occurs in connection 

 with the sheathing petiole, as in the GraminefC and Cyperaceje, 

 or where there is evidence that there has been a sheathing petiole 

 which has disappeared by degeneration, leaving the ligule axillary 

 as in some of the Naiadaceae which we shall presently consider. 



When the ligule has developed sufficiently to require special 

 support, it is supplied by the introduction of vascular bundles. 

 These bundles have their origin most frequently' as tangential 

 branches of the main leaf-bundles at their point of passage from 

 the sheathing petiole into the true petiole, or, where the latter is 

 undeveloped as in tiie grasses, into the blade. This mode of ori- 

 gin of the ligular bundles is seen in some of the tropical grasses 

 and in the ligular portion of the stipule of the Naiadaceae and the 

 ochrea of the Polygonacese. Richardia shows an exceptional ve- 

 nation of the ligule. 



The best marked examples of the sheathing petiole among the 

 Monocotyledonesare found in the AraceiC, the C3'peraceiii and the 

 Gramineje. If we examine a developing plant of the common 

 hot-house calla {Richardia Africana Kuntii.), the first leaf (fig. 

 25) is seen to be a short, broad sheath, the second (fig. 26) has in- 

 creased to a considerable length and the apical and axial tissues 



