Tin FOREST & ORNAMENTAL TREES 257 



infested with fungus mycelium, which penetrates 

 between and into the cells, but which does not, to all 

 appearance, seriously affect the vitality of the plants. 

 These organisms have been the subject of considerable 

 study, and many students of botany believe that there is 

 a definite symbiotic relationship between the two plants ; 

 that the fungus receives certain food substances and in 

 turn performs certain services for the host plant. This 

 has been disputed, but it nevertheless means that the 

 presence of a fungus on the roots of a plant is not 

 necessarily a cause of disease. 



Nematodes. These little pests, which are known to 

 be so widely distributed with reference to both flora 

 and locality, are known to attack coffee and orange 

 trees. They doubtless attack many of our forest, orchard, 

 and ornamental trees. They are not only the cause of 

 serious injuries, but they facilitate the attacks of many 

 root fungi. They can be guarded against in the nursery 

 by the proper rotation of crops, and orchard and 

 ornamental trees can be protected to some extent by 

 the use of carbon bisulphide. 



Pruning. No hard and fast rules can be given for 

 pruning any tree, but there are certain precautions that 

 should be observed in making the cuts in pruning. All 

 cuts should be made in such a way as to make a wound 

 that will heal easily. All cuts should be made parallel to 

 the trunk or limb from which a branch is cut. If large 

 limbs are to be cut, an undercut should first be made, in 

 order that the weight of the limb when being severed 

 does not cause it to fall before the cut is completed, and 

 in so doing to tear the bark down on the trunk. Such 

 wounds are very common and are dangerous sources of 

 disease infection. Such wounds are entirely avoidable, 

 and are always an ear-mark of careless pruning (Figs. 

 84, 85). 



Wounds under two inches in diameter usually heal 

 readily without further care, providing the cut has been 

 perfectly made. Wounds of larger size than two inches 

 should be painted over so as to prevent entrance of 



8 



