Special Morphology and Classification. 307 
largest, and the pits themselves always open; in other , 
cases they are always closed. When the walls of the vessels 
are also thickened spirally, as is the case in the so-called 
tracheides (Fig. 35, p. 20), this thickening sometimes dis- 
appears at those parts of the vessels which adjoin the xylem- 
parenchyma or medullary rays; but remains, the pits 
diminishing greatly in number or altogether disappearing, 
when the tracheides are surrounded by strongly thickened 
wood-cells. As regards the size or diameter of the vessels, 
those of the first annual ring are narrower than the succeed- 
ing ones ; and this is especially the case in the large vessels 
of the spring-wood. In the oak, for example, the vessels do 
not attain their full size before the sixth and succeeding 
rings. The vessels become lignified at an early period, and 
are then filled with air; it is only when young that they 
contain sap. When, in a few cases, substances of the nature 
of gum, resin, or latex occur in them, asin the cherry, these 
have passed over into them from adjoining elements of the 
tissue, but have not been formed in them. 
The whole of the elements of the xylem need not be 
present. With regard to their relative position, the separate 
elements are arranged in rows or groups, or irregularly 
without any definite arrangement ; but even then they are 
distributed in such a characteristic manner that anyone with. 
experience can determine from a small fragment of wood 
the genus, or not unfrequently even the species, from which 
it was taken (Fig. 475). The inner portion of the wood, the 
cells of which have become firmer and stronger from their 
walls having undergone lignification for a longer period, is 
called the duramen, in contradistinction to the alburnum, 
or younger layers, the walls of which are less thickened and 
lignified, and which still contain sap. When a very irre- 
gular or twisted course of the separate elements of the wood 
is caused by branches, or buds, or any similar cause, a very 
peculiar appearance is imparted to those spots, and they are 
termed nots. | 
