190 MEDULLARY RAYS. [BOOK 11. 



wounded medullary rays, although no such exudation is else- 

 where visible. In endogens, in which there appears less 

 necessity for maintaining a communication between the centre 

 and circumference, there are no special medullary rays. 

 These rays also serve to bind firmly together the whole of 

 the internal and external parts of a stem, and they give the 

 peculiar character by which the wood of neighbouring species 

 may be distinguished. If plants had no medullary rays, their 

 wood would probably be, in nearly allied species, undistin- 

 guishable ; for we are scarcely aware of any appreciable dif- 

 ference in the appearance of woody or vascular tissue ; but 

 the medullary rays (the silver grain of carpenters), differing 

 in abundance, in size, and in other respects, impress cha- 

 racters upon the wood which are extremely well marked. 

 Thus, in the cultivated Cherry, the plates of the medullary 

 rays are thin, the adhesions of them to the bark are slight, 

 and hence a section of the wood of that plant has a pale, 

 smooth, homogeneous appearance ; but in the wild Cherry the 

 medullary plates are much thicker, they adhere to the bark 

 by deep broad spaces, and are arranged with great irregularity, 

 so that a section of the wood of that variety has a deeper 

 colour, and a twisted, knotty, very uneven appearance. In 

 Quercus sessiliflora the medullary rays are thin, and so 

 distant from each other that the plates of wood between them 

 do not readily break laterally into each other, if a wedge is 

 driven into the end of the trunk in the direction of its cleav- 

 age : on the contrary, the medullary rays of Quercus pedun- 

 culata are hard, and so close together that the wood may be 

 rent longitudinally without difficulty ; hence the wood of the 

 latter is the only kind that is fit for application to park 

 paling. 



As the medullary rays develope in a horizontal direction 

 only, when two trees in which they are different are grafted 

 or budded together, the wood of the stock will continue to 

 preserve its own peculiarity of grain, notwithstanding its 

 being formed by the woody matter sent down by the scion ; 

 for it is the horizontal development that gives its character 

 to the " grain of wood," and not the perpendicular pleuren- 

 chyma encased in it. 



