STRUCTURE.] PETIOLES AND STIPELS. 281 



" Formation of the common Petiole and of partial Petioles. 

 A compound leaf gradually passes from the rudimentary 

 state, similar to a simple, crenate or lobed leaf, to the form 

 of a perfect compound leaf. The toothings or lobes grow by 

 their base at the expense of the common lamina, so as to 

 form with the latter a palmate or pinnatifid leaf. The seg- 

 ments ultimately offer a distinctly limited base : they are 

 more and more separated on a large petiole, to which the 

 leaflets are attached either immediately by their base or by 

 partial petioles. I cannot state what is the precise origin of 

 these partial petioles ; I have never been able to see them 

 distinctly, before the lamina of the leaflets has begun to 

 be developed. The form of the common petiole does not 

 become distinct before the perfect development of the 

 leaflets. 



"Formation of Stipels. I have only seen this formation 

 well in Robinia hispida. When the young leaf is still like 

 an oval oblong organ, terminating by a crenel, and having 

 besides from eight to ten crenels on each margin, no trace of 

 any tumour is to be found, either in the crenels or between 

 them. But as soon as the leaf is so far advanced that the 

 leaflets with their petiolules can be seen, and that the blades 

 of these leaflets also begin to be manifested, then we find a 

 little tumour at the axil of the petiolules ; which tumours 

 eventually become stipels. We may conclude with truth, from 

 this observation, that stipels, as well as leaflets, owe their 

 origin to the common petiole, and are formed after the leaf- 

 lets. This being the case, the common petiole plays the part 

 of an axis towards the leaflets and stipels. 



The base of the common petiole of Leguminous and of 

 some other plants often has two callosities, which are exactly 

 where we should expect stipules to appear ; these callosities 

 ought, I think, to be considered as abortive stipules. 



Second Period. Development of a Leaf in General. A 

 growing leaf, successively produces new parts proceeding 

 from the axis as crenels, discs, lobes or leaflets. Arrived at 

 a certain stage, this organ undergoes no other changes than 



