^6 VEGETABLE ORGANOGRAPHY. 



what is the peculiarity of structure in th^se plants which 

 causes this disposition to roll up, so rare in Dicotyledons, 

 but so common in Monocotyledons. 



Lastly, there are some leaves of Dicotyledons, which, 

 although provided with a middle nerve, are so narrow, 

 that they cannot be folded, and lie over one another 

 without any apparent order ; this takes place in the 

 Larch, Fir, &;c. : they are termed Imbricate. Petioles 

 without limbs present an analogous disposition when they 

 are not sheathing. 



Leaves with the petiole embracing the stem for a 

 considerable extent (this comprehends most Monocotyle- 

 dons, and some Dicotyledons), present in their infancy 

 dispositions slightly differing from the preceding. Most 

 of them, which are only composed of a dilated petiole, 

 are simply curved and imbricated upon one another ; we 

 see this in the coats of the Onion, in the leaves of most 

 Liliaceae, and m the sheaths devoid of limbs, which 

 form the upper leaves of the Umbelliferas, or the in- 

 volucra of the Compositae. Those sheathes, which are 

 very narrow, are almost flat ; and the larger they are, the 

 more are they curved. 



There are some plants with sheathing petioles, in 

 which this organ has a tendency to fold back lengthways 

 iipon itself, as if it had a middle nerve, and thus takes 

 the appearance of a vertical limb formed by the appli- 

 cation of the two sides of their upper surface : this 

 happens in the Iris. Considered in their infancy, these 

 leaves are said to be Embracing, because, being alter- 

 nate, each of them embraces by its two borders the two 

 borders of the leaf which follows it. Considered as to 

 their form at the state of complete development, they are 

 named, as I have said above, Ensiform. Among Mono- 

 cotyledons with a sheathing petiole, we find several inter- 

 mediate states between those leaves with a curved or 



