THE ANGIOSPERMAE : STEMS 897 



Climbing plants which have very long, narrow stems are those in which 

 the resistance to hydraulic flow is greatest, and they are notable for the large 

 average diameter of their vessels. Poiseuille's Law of fluid flow expresses 

 the velocity thus : — 



,. TTpr* where V = velocitv of flow. 



Sir] ' /) = pressure difi^erence. 



r = radius of tube. 

 / = length of tube. 

 7} = coefficient of fluid viscosity. 



The velocity is directly proportional to the fourth power of the radius 

 and inversely proportional to the length of the path of flow. Velocity is thus 

 much more sensitive to differences of radius than of length, and the resistance 

 off'ered by a long stem can be readily compensated by the increased width of 

 the vessels. To double the radius of a vessel will increase the ffow capacity 

 by sixteen times, but to halve the length of the path will only increase the 

 flow twice. 



Owing to the irregularities of their structure the vessels may have a 

 conducting capacity which is no more than 30 to 40 per cent, of the theoretical 

 efiiciency but their comparative capacity is nevertheless governed by the 

 hydraulic laws. 



The flow of water upwards proceeds through very numerous fine channels, 

 branching and rejoining, which are formed of single vessels or of small groups 

 of vessels islanded among tracts of non-conducting parenchyma and fibre 

 cells. Among herbaceous plants the leaf trace bundles serve for conduction 

 only to the leaves to which they are connected, and there is some evidence 

 that this physiological segregation may persist downwards in the synthetic 

 bundles of the lower stem. Whether this applies to the massive secondary 

 wood in trees is doubtful. There is no positive evidence of the anatomical 

 segregation in the mass of wood of distinct conducting tracts associated with 

 individual branches. The selective flow through limited channels, which 

 may be observed when a single leaf or branch draws a coloured solution 

 through the main stem, is probably conditioned by the downward trans- 

 mission of the water tension, which is the motive force of the flow, through 

 the physically most direct path available. 



Vessels anastomose freely both tangentially and radially, so that the 

 water currents, even if confined to the vessels, may shift their paths readily 

 in either direction, even from one annual ring to another. The force of 

 transpiration is not, however, sufficient to draw water across the medullary 

 rays, in the absence of a vascular connection. 



In most woody plants the vessels only remain functional for, at most, a few 

 years after their formation, and frequently only for one year. The outermost 

 annual rings of wood are often lighter in colour and softer in texture than the 

 older wood. They form the alburnum or sap wood, while the older rings 

 compose the duramen or heart wood (Fig. 881). Only the outermost portions 

 of the sap wood may be active in conducting water, and the heart wood takes 



