ORGANS OF NtrTEITION. 73 



followed by rapid vegetation afterwards. In the trees of tropical 

 climates the zones are not so clearly defined, because there is no 

 complete season of repose in such regions, although to a certain 



Fig. 164. 



Fig. 164. Horizontal section of the stem of a Fir three years old. The 

 lisriires 1, 2, 3, refer to the annual layers of wood. la. Cavities con- 

 taining resinous secretions. 



extent the dry season here leads to a cessation of growth, but 

 the alternation 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 zones, 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 zones ; 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 tl at 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 temperate regions. In other trees again, we have only 

 one zone produced as the growth of several years, as in the 

 Cycas ; and lastly, there are instances occurring 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 two latter are however totally independent 

 of climate, but are the characteristic peculiarities of certain 

 plants, and even of entire families. 



The annual layers of different ti'ees 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 zones being broader in warm seasons 

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



