INTRODUCTION. 23 



forms — are long, open, more or less cylindrical pipes or 

 tubes, termed vessels, which rtin through long distances, 

 especially vertically, without our being able to detect 

 any stoppage or closure of their calibre. The differences 

 between the various vessels depend on much the same 

 principles as those between cells, but since the contents 

 are usually only air (in ripe timber), and the markings 

 on their walls are usually very characteristic and 

 prominent, it is the latter especially which aid us in 

 distinguishing them, and pitted, annular, spiral, and 

 scalariform vessels are names of the commonest 

 types. 



Leaving these two extreme examples of element, 

 there is found in all wood a remarkable intermediate 

 type, which consists of prismatic closed tubes, several 

 times longer than wide, and containing living contents 

 at least when young. 



This type of wood-element is the most important of 

 all, for it is the fundamental one from which all the rest 

 are derived. 



In its youngest condition — in real timber — it is 

 known as a cambium-cell, and we must here neglect the 

 fact that it was itself derived from a more primitive 

 condition. When its walls have become thickened and 

 its living contents have become exhausted, it passes 

 over into one of two structures. It either becomes a 

 fibre or a peculiar prismatic element termed a tracheid. 



Now, since the cells of the wood may be regarded 

 simply as one of these prismatic elements cut up into 

 shorter closed prisms, or boxes, and since the vessels 

 are merely pipes formed by the intercommunication 

 of a longitudinal series of these prisms, their joined ends 

 being broken through — in a certain sense very much as a 

 water-pipe might be formed from a series of elongated 



