42 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
the open nature of the spring wood as compared with that of the 
autumn growth. The latter gradually closes up, and the timber 
produced in autumn is therefore much more dense, making the 
annual growths appear in concentric zones. A good example of 
this is seen in the illustration of a transverse section of the Silver 
Fir (Fig. 2), in which the dense autumn growth is very conspicuous. 
The medullary rays are very numerous in all the species of 
coniferous plants, as will be observed on carefully examining 
the illustrations of Conifers (Figs. 1 to 7), and they are specially 
noticeable in the Larch (Fig. 4). 
In the broad-leaved trees (Figs. 8 to 24) the tissue is altogether 
different, and more compact, than that of the conifers. The con- 
centric rings contain a large number of vasa, or pores, varying in 
size, number, and position, according to the species, but in most of 
them they gradually diminish in size and number from the spring 
to the end of the autumn growth. This is particularly the case in 
the Oaks (Figs. 16 to 18) and Elms (Figs. 23 and 24); while in 
the Maples (Figs. 8 and 9), Poplars (Fig. 14), and Willows (Fig. 19) 
they are nearly uniform throughout the growth. 
The medullary rays are a distinguishing feature of many of the 
genera of broad-leaved trees. This is well shown in the structure 
of the Oak (Figs. 16 to 18), in which the rays are generally very 
large, but not so numerous as in the Maples (Figs. 8 and 9). The 
size and number of the rays differ also in most of the species. It 
would appear that the medullary system regulates to a considerable 
extent the size, number, and distribution of the vasa. If this 
supposition is correct, it may throw some light on the commercial 
value of the timber of different varieties of the oak and other 
trees. This matter will be more easily understood by a reference to 
the illustrations of the transverse sections, especially if the observer 
has a practical knowledge of timber. If the structure of the timber 
of Quercus Robur (Fig. 18) be contrasted with that of Quercus 
Cerris (Fig. 16), and the latter with the wood of Poplar (Fig. 14) 
or Willow (Fig. 19), it may form a key to the cause of the longevity 
of certain trees over that of others grown under similar circum- 
stances. To the writer it is very evident that the durability of 
timber is regulated, in a general sense, by the size and number of 
the vasa and the medullary rays. In the oaks, both size and number 
of the vasa decrease with age, while the medullary ray-plates 
increase very much in size. This is not the case in the woody 
structure of the pines, willows, or poplars. 
