THE FUNCTIONS OF THE WOOD 211 



to the living cells also makes intelligible their presence in 

 such large quantity about one-fourth of the area of the 

 lumina of the tracheae. 



FUNCTIONAL DIFFERENTIATION OF TYPES OF MEDULLARY 

 RAY CELLS. 



The views advanced by Dixon and Atkins as to the 

 relationship of the medullary ray cells to those of the other 

 elements of the wood are in close agreement with those put 

 forward by Nicoloff (1911); and since the latter investiga- 

 tor arrived at them by an entirely different method the 

 microscopic examination of the tissues at intervals during 

 the year it may not be devoid of interest to quote from 

 his work in extenso. Nicoloff, moreover, draws attention 

 to specialization of the cells of the rays : 



" When the physiological role of the medullary rays is 

 being considered, it must be studied with due regard to the 

 relation of this tissue to its surroundings. It has been 

 usual to regard the medullary rays as vertical plates of 

 cells running in a radial direction in the stems, and facili- 

 tating the transport of nutritive materials from the exterior 

 to the interior, and vice versa. Such a definition of their 

 functions is incomplete. It is true that they do in fact 

 transmit nutritive materials in a radial direction from the 

 exterior the sieve tubes to the elements of the wood. 

 The importance of this role has been much more apparent 

 since Fischer's researches, which showed how great a part 

 the vessels play in the upward conveyance of reserves. 

 Now, it is known that the vessels themselves draw these 

 reserves from the rays, or else from the other elements of 

 the wood, which nPturn draw them from the rays. Thus 

 it is possible to establish a cycle of nutritive substances, in 

 which the starting-point is the centres of assimilation 

 taken collectively, The assimilates enter the sieve tubes 



