38 THE STORY OF PLANT LIFE 



The vessels in the tracheal tubes are thickened to 

 prevent the extra pressure from rupturing them. 

 Between these spiral and annular rings the vessels 

 are open to feed the tissues laterally. These tubes 

 are wide where produced near the growing points, 

 and possess pitted vessels further back, where the 

 conditions of growth are more fixed. The tracheal 

 tubes ramify in the stem and pass out to the 

 branches and leaves, and at every few internodes 

 join up, like arteries. Thus all parts of the plant 

 are fed simultaneously at any level. 



The tracheids are closed cells with thin spaces or 

 bordered pits through which a solution can find its 

 way. They are less freely conducting than the 

 tracheal tubes, which carry water upward, the 

 tracheids dispersing it laterally but eventually upward 

 or for short distances, but in some Gymnosperms 

 only (or mainly) tracheids occur without tracheal 

 tubes. 



In travelling round a stem the water need not 

 enter the medullary rays, as they are not continuous 

 vertically, but, as some are for water-carriage, some 

 for food-conduction, the pits enter the former, and 

 so effect a radial, and at the same time a vertical, 

 circulation. In the rings of growth in a tree there is 

 sapwood and heartwood. The former carries water, 

 and the latter is filled with food material, resin, etc. 

 Some trees possess little sapwood. The wood of the 

 year is the sapwood, and is most influential as a rule 

 in conducting water. 



