CONSTRICTION OF TWIGS BY THE BAG WORM. 163 
further interest to note that the very last wood cells formed 
before all growth ceases still have all the characteristics of 
wood fiber; they are smaller, to be sure, but there is no 
mistaking them for anything but typical wood cells. Im- 
mediately between the last wood cells formed and the band, 
will be found the very much compressed and shriveled 
remnants of the original bark, which in the majority of 
cases has been so thoroughly compressed that it appears as 
a dark, opaque line, in which no structure can be dis- 
cerned. 
The influence which pressure has upon the development 
of wood tissue was first investigated by Sachs (8) and 
DeVries(9). Sachs, in an attempt to explain the cause of 
the formation of spring and fall wood in trees, assumed 
that the cambium is acted upon by a definite pressure exerted 
by the bark, and that this pressure increases from the 
spring to the fall, equalizing itself during the winter; he 
took the formation of cracks in the bark in the spring as 
an evidence of the growth energy of the woody cylinder 
overcoming the bark pressure acting against it. DeVries, 
in an endeavor to explain the causes of spring and fall 
wood, made a series of slits in the bark in the fall, as a 
result of which he noted the formation of spring wood. 
This he explains by imagining that the bark pressure was 
destroyed because of his making slits, and consequently 
large-lumened cells were formed. He likewise placed a 
number of ligatures on branches inthe spring and observed 
the formation of summer wood under these ligatures. He 
concludes from these experiments that the radial diameter 
of the wood elements is determined by the bark pressure 
exerted during their formation; and furthermore that the 
number and size of the vessels in any one wood ring are 
determined by the bark pressure which happens to be 
experienced at the time of formation of this particular 
ring, higher pressures resulting in fewer vessels which are 
of smaller diameter. 
