THE ANATOMY OF STEMS. 257 



method to obtain it for minute examination. When 

 thus separated and placed under the microscope, it 

 appears to consist of 2 layers ; the outer being the 

 unorganized pellicle of true epidermis, and the inner a 

 vascular texture, composed of minute vessels which 

 terminate externally at the surface of the stem, and 

 internally in the cellular integument. These are, ap- 

 parently, annular vessels with oblong pores, which 

 probably perform the office of exhalents or of absorb- 

 ents. Such is the cuticular portion of the bark of the 

 Horse Chesnut ; but the structure of this part is not 

 uniform in all the woody stems of this class. In that of 

 the Pear (Pyrus communis), it consists rather of trans- 

 verse cells than of vessels, the outer series of which is 

 covered by the real epidermis : this is the case also 

 in the lesser Periwinkle (Vinca minor), in which there 

 are 3 series of such cells ; in the Laburnum ( Cytisus 

 Laburnum), it is composed of the epidermis simply 

 covering a layer of an irregularly cellular or spon°y 

 character. These and similar varieties in the struc- 

 ture of the cuticle account for the want of coincidence 

 in the descriptions of authors. 



The true epidermis or exterior layer of the cuticle 

 is necessarily cribriform, whether it act as an exhal- 

 ing or an absorbing surface ; and the manner in which 

 the pores are arranged, does not differ less, in differ- 

 ent plants, than the structure of the interior layer. It 

 is also frequently studded with hairs, glands, and 

 prickles. In young and succulent shoots, the cuticle 

 is generally almost colorless, and semi-transparent, 

 transmitting the green color of the exterior part of the 

 cellular integument over which it lies ; but it becomes 

 opaque or colored by age, or rather, on losing its vi- 

 tality ; for, as it is annually reproduced, the old layer, 

 if it does not fall off, cracks and is pushed outwards by 

 the increase of the diameter of the stem ; and the aecu- 

 22* 



