230 



AMERICAN FORESTRY 



color to realize how delicate and sensitive the cam- 

 bium is. 



The live bark is made up of a number of different 

 kinds of cells each having their own functions. Some are 

 long, thick-walled and tough to give the bark the neces- 

 sary strength. Others are specially formed to conduct 

 the plant foods manufactured in the leaves down to the 

 roots and trunk, where they may be used or stored. 

 There are grit-cells to impede the progress of boring 

 insects, and layers of succulent active cells which, when 

 the time comes, produce layers of cork. These layers of 

 cork are impervious to water. The new layers of cork 

 repeatedly cut off, from water and food, eventually pro- 

 duce the patches of dead bark. 



The dead bark acts as a mechanical protection to the 

 vital parts of the tree s It is constantly sloughing off 

 and being renewed from within. The fungi which attack 

 it are effectualy excluded from the live bark by the inner 

 cork layer. 



The fungi which attack the live bark and cambium 

 are diseases of the active succulent tissues. They are 

 known as bark diseases. They cause the death of the 

 inner bark and cambium and the separation of the bark 

 from the wood. These fungi spread most rapidly 

 through the cambium, because it is made up entirely of 

 delicate, thin-walled, active cells. In the live bark, their 

 progress is impeded by the woody fibers and the grit 

 cells. Once they reach the cambium they use this vital 

 layer, in which all new growth of wood and bark orig- 

 inates, as their line of communication and base of opera- 

 tions and strike out from it into the adjacent tissues as 

 they spread up and down and around the stem. As they 

 spread around the stem, they cut the connections which 

 carry the food compounded in the leaves down to the 

 roots. The roots dependent upon the diseased portions 

 of the bark are starved out and cease to gather moisture 

 for the parts of the tree which they formerly supplied 

 with water. As in the case of the sap rots, when a bark 

 disease encircles the trunk the death of the entire tree 

 results. 



All of these fungi, heart rots, sap rots, bark diseases, 

 and those which act in more than one of these capacities 

 are carried from tree to tree by microscopic bodies called 

 spores. The spores are the seeds of the fungi, dreat 

 numbers of these spores are produced, and many of them 

 drift about in the air without finding a good place to 

 germinate. All the heart rots and sap rots fail to get 

 a foothold unless they settle upon the wood exposed by 

 a wound in the bark. The disease of the cambium and 

 live bark must find an entrance which reaches past the 

 inner cork layer and leaves these delicate tissues without 

 effective protection. Thus every unprotected wound 

 greatly multiplies the chances for the development of 

 one or another fungous diseases. 



For this reason, the arborist protects all openings in 

 the bark of the trees by disinfecting them and water- 

 proofing them as well as |x>ssible. Coal tar has disin- 

 fectant as well as waterproofing properties so that it is 

 generally used for these purposes. More effective com- 

 pounds are being sought for each of these uses. In any 



case, the protection should be inspected at least annually 

 and renewed at intervals until the new growth or callus 

 which starts from the cambium around the edges of the 

 wounds closes them. 



Where it is worth while to take these precautions 

 against invasion by fungi, it is worth while to see to it 

 that neither "tree doctors" nor linemen are allowed to 

 clamber over the trees and poke the bark full of little 

 cuplike holes especially suited to catch the spores of fungi 

 and to conduct them directly to the susceptible tissues. 

 Spur marks are shaped like three-cornered funnels, with 

 the point down, and usually reach to the sapwood. 



PROGRESS OF DISEASE 



Chestnut log showing rings of bark-disease fungus about spur-mark 

 from which the growth started. Each ring marks the limit 

 of a season's growth. 



Xot very often, but still in an appreciable number of 

 cases, spurs open the way for heart rot. Figure 1 shows 

 a wound in the bark of a Norway maple caused by spurs. 

 The season during which the spurring occurred was evi- 

 dently a dry one, for the cambium dried out around the 

 wounds and the process of healing started before any 

 bark or sapwood disease got a foothold. The exposure 

 of the wood caused it to dry out, shrink and crack at 

 the surface. The crack exposed still more wood to 

 drying out. The drying out of the live wood kills it. 

 Heart rots live on dead wood. Thus do spurs open the 

 way for heart rot. 



The spur wounds that happen to result in the drying 

 back of the cambium without immediate fungous infec- 

 tion have in them the probability of other bad results. 

 While working on a fine old elm on which the owner had 

 previously spent considerable money, an arborist noticed 

 numbers of light-colored streaks like those which are 

 common below old wounds, but which at a little distance 

 appeared to come out of sound bark. These streaks 

 proved to come from pockets under the dead bark caused 

 by the drying back of the cambium around spur wounds. 



