116 THE VEGETABLE WORLD. 
continues from spring to autumn, and consequently under climatic 
influences very different. The great vessels are formed at the 
season when the sap is most active in its circulation ; then come 
vessels of smaller calibre and much more numerous. Towards the 
end of the year, when vegetation is less active, ligneous fibres are 
formed. It follows from these differences between the wood of 
spring and of the autumn, that it is very easy to distinguish the 
passage from the former to the latter, and that the different 
annual formations appear consequently as so many concentric 
zones on the horizontal section of the tree. To continue, it is by 
the operation of the generating zone that the tree increases im 
circumference. If the annual growth of a tree were equal during 
all the time of its vegetation, there would be no line of demarca- 
tion between each annual period, and consequently no concentric 
zones. It is thus with trees in some hot countries, where vegeta- 
tion is neither checked nor accelerated by change of temperature. 
e age of a tree is not there, as in temperate countries, to be 
found inscribed upon its stem. | 
We must here, however, offer two important remarks. The 
new wood alone participates in the nature of the exterior parts of 
the ligneous circle; the medullary sheath is not renewed. On the 
other hand, new cells are constantly formed by means of the gene- 
rating zone, in default of which the growth would cease as soon 
as all the cells of this zone were transformed into the elements of 
new wood and bark. 
