SYMPTOMS OF DISEASE. 21 



and even into the branches, causing the formation of 

 " witches' brooms." These are alterations in the nor- 

 mal system of branching peculiar to every species of 

 tree, and are due to the stimulation caused by the para- 

 site growing in the tissues of the branches or twigs. 

 Witches' brooms are very common on wild cherries and 

 plums, where they are caused by a fungus. Some 

 witches' brooms (on conifers) are produced by mistle- 

 toes (p. 56). Occasionally, however, their formation 

 is due to excess of nourishment. This is the case 

 with the rather rare witches' brooms on yellow pine, 

 caused by Peridermium knots, which " girdle " the 

 branch and stop the downward flow of the food pre- 

 pared in the needles above (p. 39). Other witches' 

 brooms are caused by mites or insects. 



Toadstools or conchs (see p. 28 and PI. V and others) 

 on trunks and limbs indicate the presence of parasitic 

 fungi and rot in the wood. Until they develop, no out- 

 ward sign of disease may be visible. Diseases of the 

 trunk or limbs may affect either the sapwood or the 

 heartwood, or both. Disease of the sapwood of conif- 

 erous trees is often indicated by a heavy flow of resin 

 from the bark. Resin flow on the butt of the tree 

 points to disease of the root collar and the roots. In 

 cases of this kind we will often find white mycelium 

 (see p. 25) under the bark or between the bark scales of 

 the butt (see pp. 42 and 53). On the trunk, where it is 

 not clearly explained by a wound, it is always more or 

 less suspicious. 



It is well to be on the lookout for any abnormal ap- 

 pearance of the trunk. Cat faces, burls, thick protrud- 

 ing knots, deep knot holes, and abnormal resin flow 



