288 DEVELOPMENT OF PETIOLES. [BOOK i. 



nascent stipules is originally completely open at the point 

 opposite the base of a leaf, whilst it only offers a notch at the 

 point which is applied at the base of a leaf. This notch 

 becomes an incision by the growth of the edges, which termi- 

 nate in points, at the same time that the rest of the rim rises 

 on the axis ; nevertheless remaining open on one side. An 

 envelope, which finally completely covers the axis, is the 

 result, the incision remaining always, whilst the open side is 

 closed in consequence of the growing together of the edges. 

 These stipules, called interpetiolar, are placed between the 

 petiole and the axis ; they completely envelope, during a cer- 

 tain time, the axis and the less advanced leaves. 



The transformations of the petiole caused byits development 

 are of little importance, in consequence of the uniformity of 

 this organ, and of its extension taking place almost entirely 

 in a longitudinal direction. In a nascent leaf the petiole is 

 generally of no distinct form, and even when it exists from 

 almost the very beginning, it is very ill-defined compared 

 with what it subsequently becomes. Measurements have 

 proved that its principal development is from above down- 

 wards. Its development modifies very considerably the 

 position of the stipules relatively to the blade of the leaf. 

 Stipules are originally inserted on the petiole along the whole 

 of their inner side, whilst their outer side is applied to the 

 axis, and their base in common with the petiole springs from 

 the axis. Now, the extension of the petiole taking place 

 principally at its summit, the blade of the leaf is separated 

 more and more from the stipules, the base of which is in no 

 way displaced ; and the more the petiole raises the blade, the 

 further the latter gets from the stipules, which finally become 

 totally distinct organs. This is very striking in Liriodendron 

 tulipifera. The leaf, including the stipules of this plant, is 

 originally nearly lyrate, with a large opaque line where the 

 midrib is afterwards found. A portion of the petiole about 

 half a millimetre long, afterwards appears at the spot where 

 the furrow is deepest, and separates the edge of the lamina 

 from that of the stipules. When, however, the leaf is com- 

 pletely formed, the length of the petiole is at least sixty-five 

 millimetres. 



