200 DISEASE IX PLANTS. 



section is the formation of a callus from the cam- 

 bium, phloem and cortex, which begins to rise as 

 a circular occluding rim round the wood. The 

 transpiration current in the trunk, however, is not 

 deflected into the 12 inches or so of amputated 

 branch, because there are no leaves to draw the 

 water up it, and so the stump dries up and the 

 cortex and cambium die back to the base, leaving 

 the dead wood covered with shrivelled cortical 

 tissues only. This dead stump gradually rots 

 under the action of wet, fungi, and bacteria, and 

 since the pith and heart-wood afford a ready 

 passage of the rot-organisms and their products 

 into the heart of the trunk, we find in a few years 

 a mere stump of touch-wood and decayed bark, 

 which falls out at the insertion like a decayed tooth, 

 leaving a rotten hole in the side of the trunk. 



If, however, instead of allowing the basal part of 

 the amputated branch to protrude as a stump, we cut 

 it off close to the stem, and shave the section flush 

 withthenormal surfaceof the latter, the callus formed 

 by the cambium, etc., rapidly grows over the sur- 

 face, and soon forms a layer of cambium continuous 

 with that of the rest of the stem. The wound 

 heals, in fact, much as if it were a strip-wound, and 

 beyond a slight prominence for a year or two no 

 signs are visible from the outside after the occlu- 

 sion. Of course these matters depend on the 

 relative thickness of branch and stem, and if much 

 wood is exposed the dangers of rot and a resulting 

 hollow in the stem are increased. It is interesting 

 to note how much thicker the callus lips are at the 



