80 MISSOURI BOTANICAL GARDEN. 
cells, presently to be described, except that their walls are 
somewhat thicker,—sometimes much thicker,—and marked, 
even on the tangential sides, with bordered pits very different 
from the simple pits of the libriform cells and parenchyma. 
Their walls not infrequently show a spiral striation which 
in its most marked form consists in a fine acute spiral ridg- 
ing of the inner surface of the cell, the ridges, however, 
differing from those of the spiral vessels in being wedge 
shaped with a broad base, whereas in the vessels they are 
of a generally round section, and attached by a narrow 
base, which is easily broken, so that they can be uncoiled 
from within the lumen. 
Except for these masses of vessels with their accompa- 
niment, and a layer of cells presently to be described, at 
the limits of each year’s growth, the wood consists of 
libriform or ordinary wood cells, which differ from the 
tracheides in possessing on the radial walls very small, 
obliquely crossing, not evidently bordered pits, scattered or 
irregularly grouped. They are fusiform or, more accurately, 
obliquely truncated at the ends, which overlap in such a 
way as to be very evident in tangential sections while much 
less obvious in radial sections. Their length is usually 
fifteen to twenty times their diameter, the measurements as 
a rule lying between 15 X275 and 20X375. Now and then 
these libriform cells are found with transverse partitions 
running directly across, one or two to each cell, a condition 
that has been observed frequently in other woods. While 
striation is much less evident than in some of the tracheides, 
it is sometimes to be seen. 
Rings indicative of the annual (or more accurately 
periodic) growth in thickness of the stem, though nearly 
invisible to the naked eye, are evident on an examination 
of the wood under a lens, being partly caused by the 
occurrence of a broken row of vernal ducts, and in part by 
a gradual reduction in the radial diameter of a few rows of 
libriform cells formed toward the end of the growing sea- 
son, whereas those first formed the next spring are of 
16 
