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 wood 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 Tries 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 Linie 
von Pinus silvestris L., Sitzungsber. Naturfor. Ges. Dorpat 6: 147-57. 
1884. 
