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1910] PENNINGTON—LONGITUDINAL COMPRESSION 273 
pine trunks, although there were exceptions to the general rule. 
WiesNER (36) has attempted to explain eccentric growth in 
branches by attributing it to unequal size and distribution of the 
leaves. 
In accordance with the observations given above, it was found 
necessary to keep in mind constantly the correlation between the 
growth of mechanical tissue and the growth of other parts of the 
plant. Locust shoots, which at one time were apparently equal 
and growing under equal conditions, were found to develop at very 
unequal rates, apparently because of differences in the amount of. 
nutriment stored in the roots from which the separate shoots had 
sprung. 
The differences in herbaceous stems under different conditions 
are perhaps best illustrated by sunflowers which were grown in the 
greenhouse and plants of the same kind and age which were grown 
in the garden. The tall, slender greenhouse plants at the end of 
four weeks had such poorly developed xylem cylinders that many of 
the stems were not able to support the weight of the tops, while 
the garden plants in the same time had become strong and robust, 
with well-developed xylem cylinders. This is, of course, an extreme _ 
case. Many plants, however, which were grown under apparently 
the same conditions had to be rejected because of abnormalities 
or differences in growth. By selecting plants which were as nearly 
uniform as could be obtained, and by keeping all conditions of 
growth as nearly equal as possible, it was thought that accurate . 
results could be obtained, especially since the possibility of a per-~ 
sonal error was also excluded. : 
MECHANICAL EFFECTS OF COMPRESSION 
The textbooks assert that pressure, or compression, retards | 
stowth and may completely stop it if the pressure is great enough 
(PEIRCE 24, Jost 15, PFEFFER 26). It seems no more necessary 
to make this assertion than to assert that heat, for example, may 
retard growth or even kill the plant if-the temperature be high 
enough. It is of importance, however, to know whether retardation © 
'S proportional to the pressure, and whether changes take place in 
the cell or in its walls. 
