84 ANNUAL RINGS OF THE WOOD. 
only be distinguished by the smaller size of the wood-cells on 
the outside of each, which appearance is caused by their dimin- 
ished growth towards the end of the season. 
The distinction between the annual rings is always most 
evident in trees growing in temperate and cold climates, where 
there is a more or less lengthened winter in which no growth 
takes place, followed by rapid vegetation afterwards in the 
spring and other seasons. In the trees of tropical climates the 
rings are not so clearly defined, because there is no complete 
season of repose in such regions, although to a certain extent 
the dry season here leads to a cessation of growth, but the alter- 
nation of the growing season and that of rest is not so well 
marked as in colder climates. As alternations of growth and 
seasons of repose may thus be shown to produce the appearance 
of annual rings, we can readily understand that if a plant were 
submitted to such influences several times in a single year it 
would produce a corresponding number of rings ; and this does 
really occur in some plants of temperate climates, particularly 
in those which are herbaceous, where growth is more rapid than 
in hard-wooded perennial plants, so that the influence of such 
alternations is more evident. In tropical climates the production 
of two or more rings in a year is probably even more frequent 
than in temperate regions. In other trees, again, we have only 
one ring produced as the growth of several years, as in the 
Cycas ; and lastly, there are instances occurring in which no 
annual rings are formed, but the wood forms a uniform mass 
whatever be the age of the plant, as in certain species of Cacti. 
Such appearances as the two latter are, however, totally inde- 
pendent of climate, but are the characteristic peculiarities of 
certain plants, and even of entire natural orders. 
The annual layers of different trees vary much in thickness, 
thus they are much broader in soft woods which grow rapidly, 
than in those which are harder and of slower growth. The in- 
fluence of different seasons, again, will cause even the same tree 
to vary in this respect, the ‘rings being broader in warm seasons 
than in cold ones, and hence we find the trees as we approach the 
poles have very narrow annual rings. The influence of soil and 
other circumstances will also materially affect the thickness of 
the annual rings in the same tree. We find also that the same 
ring will vary in diameter at different parts, so that the pith, 
instead of being in the centre of the wood, is more or less eccen- 
tric, owing to the rings being thicker on one side than on the 
other. This irregular thickness of the different parts of the 
annual rings is owing to several causes, but the greater growth 
on one side is chiefly due to the fact of its being more exposed 
to light and air than the other. 
The annual rings also vary in thickness in the same tree, 
according to the age of that tree. Thus when a tree is in full 
vigour it, will form larger rings than when that period is past, 
