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94 MISSOURI BOTANICAL GARDEN BULLETIN 
retain the filling, the upper and lower ends should be slanted 
downward to form water sheds, and the entire opening 
should have an oval form to facilitate healing. Then the 
cavity is braced by running 34—34-inch machine bolts from 
the side of the opening diagonally through the back of the 
tree. These are inserted every 12-18 inches, for the pur- 
pose of preventing the filling from cracking off from the 
sides, due to torsional stresses upon the trunk. The hole 
for the bolt should be bored to be of the same diameter as 
the bolt, while a square opening should be made in the 
bark for the nut, deep enough to sink the bolt head under 
the bark so that it will be eventually grown over. The other 
side has a circular opening to allow for a washer. The 
cavity and bolts are then creosoted and later fumigated to 
kill any borers which may be in the sound wood. For this 
purpose a teaspoonful of carbon bisulphide is used to every 
cubic foot of cavity, the entire opening being covered with 
cloth or tar paper, and a piece of cotton saturated with the 
solution dropped into the cavity. If the cloth does not allow 
the fumes to escape, the borers are killed over night. The 
cavity is then ready for filling. 
_The material most commonly employed is concrete, 
either a dry or wet mixture of 1 to 4. The best Portland 
cement should be used, coarse sand, and ¥%-inch gravel. 
No reinforcement is necessary, except the bolts, as it will 
not prevent the cracking which is bound to occur because 
of the swaying of the tree. Moreover, the strengthening 
effect of the reinforcement is questionable because an in- 
elastic substance like concrete cannot strengthen the elastic 
wood. The dry mixture is the easier to use. The concrete 
should be mixed so that it will not crumble, and is then 
laid in the cavity and brought out to within 4 inch of 
the cambium layer. It is very important that the filling 
should be below the cambium to permit it to grow over 
eventually. The surface of the filling may be gone over 
with mortar to give it a smooth finish, and after drying 
out a coat of tar or fluxed asphalt is applied to make the 
concrete waterproof. The “dry” concrete has the disad- 
vantage of disintegrating, as air spaces are left which are 
penetrated by water and the cement leaks out. The “wet” 
concrete method necessitates the use of a form. This may 
be made of wood, but that is not economical and requires 
driving nails into the bark. A wire netting is better, which 
may be inserted into the cavity, the concrete poured in and 
then faced with mortar made of a mixture of cement, lime, 
sand, and water (1 part of cement to 2 of lime and sand). 
Still another method requires the use of oil-cloth. Two 
