544 cook: morphology and evolution of leaves 



leaf, or by moving the pinnae farther up the rachis of a com- 

 pound leaf, as in the pinnate palms. 



SHEATH PETIOLES AND BLADE PETIOLES 



The recognition of the two kinds of petioles as representing 

 distinct morphological elements makes it necessary to have more 

 convenient ways of designating the two classes of organs to 

 which the word petiole has been applied indiscriminately. The 

 use of such terms as sheath petiole and blade petiole would afford 

 a way of indicating the distinction. Sheath petioles would be 

 understood as those that represent specializations of the primi- 

 tive sheath, and blade petioles as specializations of the blade or 

 midrib of the leaf. 



Other terms that might be used are infraligular for the sheath 

 petioles and ultraligular for the blade petioles, in allusion to the 

 differences of position in relation of the ligule. It might be 

 objected that ligules are confined to a few families, but the terms 

 would still serve to indicate the homologies of the parts to those 

 of plants whose possession of ligules gives the most definite basis 

 for the distinction between the two kinds of petioles. 



THE FOOT AS A NEW ORGAN 



For general descriptive purposes and especially for dealing 

 with plants in which the resemblance to the primitive sheath 

 has entirely disappeared, it may be simpler to treat the petiole- 

 like sheath element as a new organ not formally distinguished 

 hitherto. On this basis instead of sheath petiole or infraligular 

 petiole, a single word like foot, in Latin descriptions pes, might 

 be used. This would have the advantage of leaving the term 

 petiole as nearly as possible in its present signification, which 

 would need to be modified only in those cases where the so-called 

 petiole might be found to represent the sheath element. No 

 doubt there are many plants where not only the stipules are lack- 

 ing, but also the foot. 



From this point of view it would be possible in dealing with 

 different families of plants to define the foot in any way that 

 might be most convenient, as a thickened segment of the primi- 

 tive sheath, as the element of the primitive sheath that supports 



