STRUCTURE OF WOOD IN GYMNOSPERMS. 83 
plant continues to live. It is in consequence of each succeeding 
layer of wood being thus deposited on the outside of those of 
the previous years, that these stems are called exogenous. In 
the stems of the Conifer and most other Gymnosperms, as those 
of the Fir, Yew, and Cypress, the annual rings of wood which 
are well marked (jig. 186), instead of being formed of ordinary 
woody tissue, and pitted vessels, consist essentially of wood- 
cells, with large bordered pits (see pages 44 and 50). 
The pitted vessels, which as we have seen form an essential 
portion of the annual layers of the wood of all exogenous stems, 
except those of the Gymnospermia, as mentioned above, are so 
large in the Oak, Ash, and other plants, that they may readily 
be seen by the naked eye upon making a transverse section of 
the wood of such trees ; and in all cases, upon examining under 
the microscope a transverse slice of any common exogenous 
stem, the pitted vessels may be at once distinguished from the 
Fic. 186. 
epee 
peer aerate at 
=, ~ = “cr ote see 
cons SE 
. See ae pee 
ge 
twee 
oan 
Std 
Fig. 186. Transverse section of the stem of a Fir three years old. The 
figures 1, 2, 3, refer to the annual layers of wood. Ja, la. Cavities contain- 
ing oleo-resinous secretions (7eceptacles of secretion). 
wood-cells by the larger size of their openings (figs. 183, v, v, 7 
and 185, A, b, b, b). 
But in those Gymnosperms where the wood is made up, as just 
noticed, of disc-bearing woody tissue, or cells with large bordered 
pits, though the openings of the cells are larger than those of 
ordinary woody tissue, they will be observed to be nearly of the 
same size, but at the same time those formed earliest in the 
year in each ring are larger and have thinner walls than those 
which have been formed at the end of the year (jig. 186). The 
pitted vessels in ordinary trees are also commonly more abun- 
dant on the inner part of each annual ring, the wood-cells 
forming a compact layer on the outside (fig. 185, A, c, c, c). In 
such cases the limits of each ring are accurately defined. In 
those trees which have the pitted vessels more or less diffused 
- throughout the wood-cells or woody tissue, as in the Lime and 
Maple, the rings are by no means so evident, and can then 
G 2 
