No. 474] SAP FLOW IN MAPLE 441 



only, it seems reasonable that some stimulus must be applied, and 

 since other factors are practically constant, it seems reasonable to 

 expect this stimulus to be the rising temperature. It is difficult 

 to think of any other factor among the conditions obtaining in a 

 bleeding maple tree that could give such a stimulus. But the 

 temperature changes would reach all parts of a narrow longitudinal 

 wood fiber at almost the same time, and the stimulus woukl not in 

 that case be unequal. For these reasons it seems improbable 

 that the wood fibers can take any great part in the production of 

 pressure and flow. 



This is not the case, however, with the pith rays. KxteiuHng 

 radially through the wimmI with few Uiteral pits and numerous 

 end ones, they are a(hnirably achipted for radial conduction with- 

 out much lateral loss. ( )nly at coin])aratively long inter\ als where 



The radial conduction of water in any wood is very limited, and 

 the many layers of wood fibers in this case would form an especially 

 efficient barrier between the inner and outer wood, each layer of 



suppose the ray cells more permeable to sugar at the outer or the 

 inner ends, then conditions are all suitable for the production of 

 pressure and flow. The penetration of early morning heat wonhl 

 tend to warm all of the radially elongated ray cells at the outer end 

 before the inner. Throughout the whole period of risin^' leiii- 

 perature therefore one end of each cell would be slightlx wanner 

 than the other. It is easy to conceive of this condititm acting as 

 a stimulus to cause a similar unequal permeability in all the cells. 



We may conceive of the i)hen(mienon occurring somewhat in 

 this way. * During the wiiUer montli< l)Ut little >tarcli i> convei-ted 



the stored starch is gradi'ially c.mverted. In tin. uav the sip of 

 the li^ing cell mu-^t luv.,uie hi^^hly eonceutrate.l an.l the ..Mitotic- 



become very great. 



Rising temperature, however, 1a wan 



