124 



ANATOMY. 



mouths of these vessels and fibres are very evident, as is the .relative thickness 

 of their walls : we shall return to this immediately. The cellular tissue of the 

 ring, disk, and bands, constitutes the medullary system. The medullary system 

 of the disk (M) is called the pith ; that of the ring is the cortical pith ; and the 

 radiating cellular bands (EM) are the medullary rays. The wedges of fibres and 

 vessels, separated by the rays, are the fibro-vascular system. If we now dissect 

 one of the bundles in a well-formed stem of Melon, whose duration is annual 

 (fig. 682), it is found to be tolerably stout, and completely surrounded by the cellular 

 tissue, the pith (M), bark (PC), and medullary rays (EM). Beginning from the 

 interior, we find, 1, spiral vessels (T), and opaque white fibres with thick walls; 2, 

 fibres (F) with thinner walls, and consequently larger cavities, arranged in series, 

 and altogether occupying half of the wedge ; together with annular, rayed, and 

 dotted vessels (v p), recognizable, especially the latter, by the size of their walls ; 



8, a greenish cellular tissue (c) ; 4, 



thick-walled fibres (L) like those next 



the pith, but more abundant ; 5, some 



branching (laticiferous) vessels (v L) 



with soft walls ; 6, the cortical paren- 

 chyma (PC), covered by a membrane (E) 



consisting of the epidermis and cuticle. 



In a horizontal section of the stem (fig. 



681), the tracheae (T) and fibres next the 



pith form with the neighbouring vessels 



a ring (interrupted by the medullary 



rays), which has received the collective 



name of medullary sheath ; the fibres 



outside this sheath are the woody 



fibres ; the outer fibres, separated from 

 the former by a cellular zone, and resembling those of 

 the medullary sheath, are the woody fibres of the bark ; 

 finally, the cellular zone which separates the cortical 

 from the woody fibres is called the cambium layer. In 683 Mapie. 



the Melon, this zone dies each year, together with 5g3^ l i t 1^T 





the fibro-vascular bundle, which it divides into two " 

 unequal parts ; but in a woody-stemmed, and hence perennial plant (Oak, Elder), 

 fresh layers are annually formed in the thickness of this zone, by which the 

 thickness^f the stem increases. Young branches, therefore, one or two years old 

 or more, must be examined, to trace the further development of the wood and bark. 

 A fibro-vascular bundle in a one-year-old branch of Oak, Elder, or Maple (fig. 683), 

 coincides in structure with that of the Melon stem ; but in the cortical system (P c) 

 there will be found, between the epidermis and central layer of cells, a layer of 

 close-set cubical or tabular cells (s) ; these contain no chlorophyll, are white or 

 brown, and are readily distinguishable from the subjacent cortical cells, which are 

 polyhedral, coloured by green granules, and separated by numerous interstices. This 



