4 
526 J. G. GROSSENBACHER 
had entered the level of the section from one side or the other by 
growing in length and penetrating between the ends of the neighboring 
cells. Such an idea seemed plausible because of the fact that the 
intruded cells nearest the cambium are small and no corresponding 
radial rows could be found in the cambial zone from which they could 
have arisen. The absence of a corresponding row from the phloem 
also showed that such cells present in the xylem could not have arisen 
from an eliminated cambial row. 
In case wood cells undergo longitudinal growth after arising from 
the cambium, it appeared likely that in occasional sections one ought 
to find unmated pits in cells that had been forced 
apart, for, as has been well established, many of 
the pits are derived from the cambium cells by 
division and must therefore be present on cell 
walls before gliding growth has been completed. 
As a matter of fact, several such instances were 
found, one of which is reproduced in figure i. The 
fourth cell from the left in the lower radial row (2) 
of this figure shows an unmated pit, although the 
corresponding cell of the next row (i), that evi- 
dently arose from a cambium cell at this level, 
shows no corresponding pit. Along the upper edge 
of the figure is a medullary ray and on the left are 
the last wood cells of two radial rows that had 
been differentiated from the cambium. Between 
the two radial rows, arising from the cambium at 
the left, are seen what appear to be intruding cells 
(i) mentioned above. These intruders seem to 
have forced their way between their neighbors 
at the angles, at points where one usually finds 
angular intercellular spaces. 
Longitudinal radial sections, however, furnished the most direct 
and therefore the most conclusive proof yet published to show that 
gliding growth occurs. A careful examination of many slides showed 
that the ends of wood fibers are often compressed and diverted at 
medullary rays, as had been found by Neeff. In some cases not only 
the ends of the wood cells in contact with medullary rays were flattened 
and enlarged, but the ray cells encountered were also compressed. 
In other instances the ends of the wood-cells were found diverted to 
Fig. I. Cross Sec- 
tion of A pple Wood, 
showing cells near- 
est cambium at left 
and a monoseriate 
medullary ray along 
the upper edge of 
figure, r, medullary 
ray. i and 2, regu- 
lar rows of xylem 
cells derived from 
the cambium at this 
level, i, cells in- 
truding between the 
regular rows at their 
angles. 
