44 TREES IN WINTER 



fact medullary rays seen in surface view. In the radial section 

 the annual rings are evident and the spring wood is seen to be made 

 up of hollow tubes. In the tangential section the rings^, if they 

 show at all, form Vs or ellipses due to the saw failing to cut ex- 

 actly parallel to the grain. Boards are generally sawed off from 

 parallel sides of a log so that, although the outer cut is tangential 

 and the median cut is radial most of the boards will be sections 

 intermediate between these two. The markings on the finished 

 lumber will accordingly vary in appearance. 



To many it is a surprise to learn that the large bulk of a tree 

 is dead, literally with no more life than the proverbial barn door. 

 Hollow trees are not infrequently found alive and flourishing. 

 Except for their liability to be blown over, they often do not seem to 

 be greatly handicapped by the loss of inner.wood. The older wood, 

 therefore, is mainly of service as a mechanical means of support 

 and has no part in the life activities of the tree. In some fonns 

 the inner wood as it dies, takes on a darker color and under the 

 name of heartwood is easily distinguished from the younger, 

 lighter colored sapwood toward the outside. The outer layers of 

 the bark also are dead. There remains, therefore, as the essential 

 living portion of the stem, the cambium layer with the inner bark 

 on its outer side, and the sapwood just within. 



Experiments have shown that the ascending currents which 

 carry water and» dissolved minerals up to the leaves are located in 

 the sapwood. The inner bark transports the manufactured food 

 from the leaves. If the inner bark is severed, as by "ringing," 

 the descending sap is interrupted and the roots ultimately 

 will die of starvation. Portions above the ring are still able to 

 obtain water and raw material through the channels in the sap- 

 wood so that injury brought about by girdling may not make itself 

 shown till the succeeding season. 



The cambium layer is responsible not only for the regular for- 

 mation of wood and bark but also for their regeneration. An in- 

 jury which exposes the cambium will excite this layer to increased 

 growth and a formation of wound tissue or callus will result. 

 Callus spreads gradually in all directions from the exposed cam- 

 bium where it first makes its appearance. Bark forms on its outer 

 surface as it slowly seals over the wound. Unless destructive 

 wood fungi gain entrance through the wound, the healing pro- 



