STRUCTURE.] THE PETIOLE. 295 



the latter. The development takes place in all directions, 

 and predominates in determinate directions. 



The blade of a leaf is first developed. Leafy lamellae are 

 extensions of it, whether they are equilateral or inequilateral. 



Teeth and crenels appear to be owing to the development 

 of certain series of cellules from the edge of a leaf. No trace 

 of them is to be found in very young leaves, the blades of 

 which are beginning to be formed. 



Stipules of Dicotyledons, in consequence of the great longi- 

 tudinal development of the petiole, appear as organs distinct 

 from the blade. The rapidity of their development is pro- 

 bably due to their proximity to the axis. Their blade is 

 developed, covering the axis or other organs. 



The petiole is principally developed in one direction; of 

 all the parts of a leaf, it is that which grows the most in 

 proportion to its original size. 



Although most of these views are founded on facts, yet 

 they want an absolutely certain basis, which cannot be ob- 

 tained without observing the internal life of the parenchyma 

 of the leaf and of its products. This ought to remain the 

 object of a true history of the development of leaves ; for at 

 present their successive transformations only, have been 

 observed." (Annales des Sciences). 



The PETIOLE, or leafstalk (fig. 57., a b), is what connects 



fig. 57. 



the blade with the stem, of which it was considered by 

 Linnaeus as a part. It consists of one or more bundles of 

 fibro- vascular tissue surrounded by cellular substance. It has 



