Grossenbacher Radial Growth in Trees. 59 



year's growth were often found arranged at right angles to the 

 axis, under unbroken bands. 



In the latter part of the second summer following the attach- 

 ment of the bags the portion of the twigs distad to the constric- 

 tion had much starch in the bark rays and pith, while that on 

 the basad side was practically devoid of it. 



In hard-wood trees both bark and \vood were found to have 

 continued growing under unbroken bands though welts developed 

 on both sides. The first wood cells formed under the ligatures 

 were normal but those developing afterwards had a shorter ra- 

 dial diameter and thicker walls than those under normal pres- 

 sure. The number of vessels appeared to decreaes in proportion 

 to the pressure. The wood fibers developing under high pres- 

 sure were found to have their long axis at right angles to the 

 twig or parallel with the compressing band, and the rays were 

 bent or buckled laterally unded pressure. It is held that the in- 

 creased pressure induces the formation of smaller wood cells not 

 because cambial division occurs before the cells have attained the 

 normal size but because the pressure hinders their enlargement 

 during subsequent differentiation. 



A large number of tests made to determine the breaking strain 

 of the bands from both conifers and broad-leaved trees showed it 

 to be about 40 atmospheres ; and, therefore, indicates that 

 Krabbe's experimental results showing a growth pressure of 15 

 atmospheres are too low, since von Schrenk's observations show 

 that the majority of the bag-worm ligatures are ruptured by 

 the enlarging twigs. 



An osmotic-pressure hypothesis. In a paper on the devel- 

 opment of pits in the wood cells of the Abietineae Russow 119 

 suggested another explanation of ' ' annual ' ' rings. He claimed 

 that the bark pressure hypothesis of Sachs which de Vries en- 

 deavored to support by experiment, cannot account for the oc- 

 currence of growth rings in the wood because the last phloem 

 cells of a season do not have a reduced radial diameter and on 

 account of the fact 'that two rings may be induced by defoliat- 

 ing trees. The bark-pressure hypothesis is also held to be dis- 

 credited by the occurrence of growth rings in 'the tropics where 



119 Russow, E. tiber die Entwicklung des Hoftiipfels, der Membran 

 der Holzzellen und des Jahresringes bei den Abietineen, in erster Lime 

 von Pinus silvestris L., Sitzungsber. Naturfor. Ges. Dorpat 6: 147-57. 

 1884. 



