The Nature and Origin of Stipules. 29 



and it is a matter of common observation, that the petiole is the 

 last portion of the leaf to develop ontogenetically and is therefore 

 to be regarded as the most recent part to be added phylogeneti- 

 cally. This helps to explain the common occurrence of sessile 

 and petiolate leaves even in different species of the same genus, 

 as variation more readily occurs in recent than in ancient struc- 

 tures, while on the contrar}^ it has been a matter of remark among 

 even the earlier botanists that stipules when they occur usually 

 characterize all the species of a family, an additional evidence of 

 the antiquity of their origin. 



Let us now take up, in the light of the foregoing conclusions 

 the consideration of the destiny of the primitive foliar organ as 

 it has been modified and developed in the course of evolution. 

 For convenience in making our inquiry, I would divide the 

 primitive leaf into the central-basal, axial, apical and lateral por- 

 tions. Each of these figures prominently in the evolutionary his- 

 tory of foliar organs, for from the original condition there has 

 been progressive development along several lines of varying de- 

 grees of relationship and the morphological result of the develop- 

 ment of the several parts has been quite different in the divergent 

 groups, so much so as to render the question of homology a 

 doubtful one to many minds. We shall endeavor to establish its 

 reality. 



The lamina of the leaf, as we shall see, has been developed 

 chiefly from the apical portion, usually from scarcely more than 

 a mere point, though it may also include the axial and lateral por- 

 tions. The true petiole, when present, is developed from the 

 axial portion,* the sheathing petiole from the central basal to- 

 gether with the lateral portions, stipules and structures of the 

 same signification from the lateral portions. It is with the lateral 

 portions, therefore, that we are chiefly concerned. 



With reference to the formation of stipules there are three 

 principle types of leaf-development : that in which the several 

 portions of the primitive leaf have developed together into a sim- 

 ple unappendaged blade, that in which a sheathing petiole is 

 formed with or without a ligule or ochrea, and that in which 

 stipules properly so-called are present. 



In the first and simplest case the development of all the parts 

 *SeeS. H. Vines. Text-book of Botany, 1: 49. 1894. 



