Grosseribacher Radial Growth in Trees. 57 



perhaps zero or even negative in depressions. Among the nu- 

 merous angular young twigs examined the greater pressure at 

 the angles did not prevent the development of normal spring 

 wood, but larger numbers of both spring and summer wood cells 

 were produced in the depressions than on the ridges until the 

 twig became cylindrical. 



It was found that when a tree-trunk or branch presses against 

 some non-yielding object or the bases of the component branches 

 of a forked stem press against each other, radial growth is re- 

 duced on the side of contact when the pressure has reached a 

 certain intensity and that the rays spread outward and eventu- 

 ally became parallel to the obstructing surface. The continu- 

 ance of radial growth tends to separate or pull apart the com- 

 ponents of a forked stem or widen the upper angle a branch 

 makes with its axis. Branches thus firmly pressed against each 

 other eventually fuse and the rays then come to radiate from 

 the common center and further radial growth tends to result 

 in a cylindrical, united structure. It was found that the callus 

 developing at the cut end of a twig in water also conformed to 

 the general law of the mechanics of radial growth in that its 

 cross sections become semicircular with a rough outline ; but the 

 surface becomes smooth as tension is developed by further 

 growth. When a rectangular piece of bark was cut from a tree 

 the first division of the cambium in the formation of a callus is 

 said to be by a radial wall or one at right angles to the wall 

 formed under normal conditions. Further growth and division 

 was also found to occur in accordance with the resistance to 

 growth and resulted in a structure having its center at the place 

 where the first cambial divisions took place. The rays in the 

 bark on both sides of the piece cut out become diverted not only 

 by the contraction of the bark at the time the piece was cut but 

 also by the lack of surface growth in the bark surrounding the 

 wound. The omission of surface growth is said to be due to the 

 lack of accustomed tangential pull formerly exercised by the ex- 

 cised piece. Growth is resumed only after the callus bark has 

 reached a tension comparable to that of the piece removed. This 

 resulted in increased radial growth in the entire region in- 

 fluenced by the wounding, as shown by a count of the number of 

 cells produced here as compared to that produced in other places. 

 When the cambium was first freed from its normal bark pres- 



