SUPPORTS 259
ring-porous wood. In tangential section the pores appear as well defined grooves
but the rays are not very distinct. The American or black walnut is darker, more
uniform in color, and not so prominently figured as the European variety; it is
more distinctly ring-porous than European walnut. Most of the pores contain
glistening tyloses. The rays are fine and inconspicuous. The wood is comparatively
free from warping and has good gluing qualities. Walnut was used to a minor
extent by all the important European schools in panel painting.
White-Wood is a name given to woods from various species of the poplar. It
is also frequently applied to tulip poplar and to bass-wood (see also Poplar).
Willow (Salix). The European willow (£. alba] furnishes a soft, light, fine-
grained wood similar in color and texture to poplar. The American black willow
(S. nigra) is similar except that the heart-wood is reddish brown. One of the most
outstanding properties of willow-wood is that it does not split easily. The wood
is diffuse-porous but annual rings are distinctly visible. It has no distinct figure
in longitudinal section. Although the records do not show that willow was in any
way extensively used in panel painting in Europe, its valuable properties must
have suggested it for that purpose.
Wood. Woods differ in many respects. Some of their characteristics can be
recognized by the unaided eye but others are so fine that they can only be studied
or recognized with the microscope. Dr Eloise Gerry of the Forest Products
Laboratory, Madison, Wisconsin, has discussed the terms used in describing
woods, the distinguishing characteristics which make the differentiation of various
woods and the identification of species possible. The following is an abridgment
of Dr Gerry's discussion of terms. First are characteristics common to woods of
the north temperate zone:
I. Heart-wood and Sap-wood. The interior wood of mature trees is usually
darker in color than that lying near the surface. The dark part is heart-wood, the
region where active growth has stopped. In the outer sap-wood, growth is still
active. Trees with characteristically colored heart-wood are black walnut, cherry,
oak, and red cedar. In some trees, however, there is little or no difference in color
between heart-wood and sap-wood.
a. Annual rings. The cross section of a tree trunk shows a ring structure.
These rings represent seasonal growth and are called * annual rings/ Each spring
wood formation begins and it continues until some time in late summer.
3. Spring-wood. The cells formed at the beginning of the growing season have
the largest cavities and the thinnest walls. They make up the spring-wood which
is the softer, lighter-colored part of the annual ring.
4. Summer-wood. The cells formed during the latter part of the growing season
have smaller cavities and thicker walls. The summer-wood is generally heavier,
darker in color, harder, stronger, and less porous than spring-wood. One layer of
spring-wood and one of summer-wood together constitute a year's growth or
annual ring.