SPRING WOOD AND AUTUMN WOOD 335 



divided up into radial wedges by the rays. The rays 

 are generally joined by tangential bands of xylem 

 parenchyma, so that the wood is divided into relatively 

 small blocks of tracheids and vessels, each in contact 

 with living parenchyma cells. It is probable that this 

 fact is related to the mechanism of conduction of water 

 and other substances through the wood, which is not 

 fully understood. Accompanying the rays are hori- 

 zontally running intercellular channels which com- 

 municate with the cortical system of intercellular spaces, 

 and thus serve to aerate the living cells of the secondary 

 xylem. Broad rays [principal rays) are often formed 

 as continuations of the primary rays (separating the 

 primary bundles) when these are narrow, and can be 

 distinguished from the narrow subordinate rays formed 

 opposite the primary bundles (Fig. 55, C). 



Annual Rings. — The well-known concentric rings seen 

 on the cut stump of a felled tree are the expression of the 

 fact that the cambium does not form perfectly uniform 

 wood throughout the year. In the early summer, when 

 its activity begins, it forms numerous and often large 

 vessels with accompanying parenchyma (" spring wood"). 

 These provide new conducting channels for taking water 

 and salts up to the new foliage. In the late summer, 

 after these channels have been provided and the foliage 

 is no longer increasing, the vessels are smaller and fewer 

 in number, and the bulk of the wood produced is often 

 composed of fibres, the water-conducting elements being 

 sometimes qiiite absent (" autumn wood "). The contrast 

 between the alternating zones of spring and autumn 

 wood gives the rings which can be counted with the naked 

 eye on the surface of a tree-stump. A check to growth, 

 for instance a severe drought at midsummer or the 

 wholesale destruction of the foliage by caterpillars, 



