ORGANS OF NUTRITION. 79 



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 tlie appearance of annual zones, we can 

 readily understand that if a plant were submitted to such in- 

 fluences several times in a single year it would produce a con-e- 

 sponding number of zones; and this does really occur in some 

 plants of temperate climates, particularly in those which are her- 

 baceous, where growth is more rapid than in hard-wooded per- 

 ennial plants, so that the influence of such alternations is more 

 evident. In tropical climates the production of two or more 

 zones in a year is probably even more frequent than in tem- 

 perate regions. In other trees again we have only one zone pro- 

 duced as the growth of several years, as in the Cycas, so that 

 in such a case the number of zones by no means corresponds 

 to the age of the tree. Lastly again, there are instances occur- 

 ring in which no annual zones 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 latter are however 

 totally independent of climate, but are the characteristic pecu- 

 liarities of some plants, and even of certain entire families. 



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 sloAver growth. The in- 

 fluence of ditFerent seasons again will cause even the same tree 

 to vary in this respect, the zones being broader in warm seasons 

 than in cold ones, and hence we find the trees as we approach 

 the poles have very narrow annual zones. The influence of soil 

 and other circumstances will also materially affect the thickness 

 of the annual zones in the same tree. We find also that the same 

 zone aWII vary in diameter at different parts, so that the pith, 

 instead of being in the centre of the wood, is more or less ec- 

 centric, owing to the zones being thicker on one side than on 

 the other. This irregular thickness of the different parts of the 

 annual zones is owing to several causes, biit the greater growth 

 on one side is chiefly due to the fact of its being more ex- 

 posed to light and air than the otiier. 



The annual zones also vary in thickness in the same tree, 

 according to the age of that tree. Thus Avhen a tree is in full 

 vigour it will form larger zones than when that period is past, 

 and it begins to get old. The age in which trees are in full 

 vigour varies according to the species ; thus the Oak, it is said, 

 will form most timber from the age of twenty to thirty, and that 

 after sixty years of age the amount formed will be much less 

 considerable. Again, in the Larch, the vigour of growth appears 

 to diminish after it is forty years of age; in the Elm after fifty 

 years; in the Beech after thirty years; in the Spruce Fir after 

 forty; and in the Yew after sixty years. Further observations 

 are required however upon these points, Avhich are of great 



