134 CUTICLE ITS NATUKE. [BOOK i. 



fig. 2. a.}-, it even consists occasionally of several layers of 

 vesicles, as in the Oleander, Cacti, and many Orchids, and it 

 varies in the density, form, and arrangement of its component 

 cells in different species according to the peculiar conditions 

 to which they are exposed in the places that naturally produce 

 them. 



External to the epidermis is a thin homogeneous membrane, 

 formed of organic mucus (see page 7) and overlying every 

 part except the stomates and the stigmatic tissue. It was first 

 observed by Adolphe Brongniart in the Cabbage-leaf, after- 

 wards by Henslow in Digitalis, and by myself in Dionsea ; it 

 has subsequently been the subject of more extended observa- 

 tions, and appears to be a universal coating, which is even 

 drawn over the hairs, as if to protect the tender cell forming 

 their interior. I have found this cuticular membrane on the 

 delicate petals of Hydrotsenia Meleagris, from which it may 

 be easily removed after maceration for a few days in spirit of 

 wine; and Ad. Brongniart succeeded in separating it from 

 the leaves of Potamogoton lucens, after very long maceration 

 in water. It is stated to be sometimes covered with a minute 

 granular appearance, the nature of which is unknown, and 

 which is not found at the lines indicating the place where the 

 cuticle was pressed upon the united sides of cells. There are 

 some good observations upon this subject by M. Adolphe 

 Brongniart (Ann.desSc. 2 ser. 1. 65.), who however imagines 

 that the cuticle overlies the stigma in Nympheea and Mira- 

 bilis. It certainly does not cover the stomates, nor such 

 glands as those found on the surface of the inside of the 

 pitchers of Nepenthes. 



Mohl regards it, with reason, as a secretion of the epidermic 

 cells, whose walls become thickened next the surface by the 

 deposit of internal layers, as happens in other cellular tissues. 

 In proof of this he observes, that when a transverse section 

 of epidermis is treated with iodine, the walls of the epidermic 

 cells, in most cases, remain uncoloured ; and only in particular 

 cases, as in Hakea pachyphylla, assume a yellow tinge. But, 

 on the other hand, in all cases a thicker or thinner layer, 

 lying on the surface of the epidermis, becomes stained by 

 iodine of a deep yellow or brown. In the epidermis of the 



