108 ORIGIN OF THE WOOD. 



From the above theory, we are cautioned against excessive 

 pruning, where we wish to obtain substantial stocks ; for all 

 other things being equal, the growth of the body of a tree will 

 be in direct proportion to the leaves, which, of course, will be 

 in proportion to the branches. If these are removed to a great 

 extent, the growth of the tree is retarded, if not otherwise in- 

 jured. 



191 1 Opposed to the above theory are some of the most dis- 

 tinguished philosophers. They suppose the vascular bundles 

 proceed from below upward, originating in the cambium cells 

 in dicotyledons, and in monocotyledons from the terminal buds. 



192. The duration of vegetables is exceedingly various. 

 Some come forth during the darkness of a single night, and 

 wither and die on the approach of day ; others go through 

 their different stages of growth to perfection in a few days, and 

 disappear. Some require the length of several months for the 

 same operation, and others of two years, while others still come 

 to perfection only after several years, and then are continued in 

 existence for indefinite periods. The latter is the case with 

 forest trees. We see nothing, theoretically considered, opposed 

 to the unlimited duration of dicotyledonous trees. Each years 

 growth may be considered a distinct individual, having in itself 

 all the elements for the production of a similar individual, 

 which, when produced, has no necessary dependence upon its 

 progenitor; since each layer of any tree has been endowed 

 during its time with all the productive functions of the individ- 

 ual. But to this unlimited duration of vegetables, nature 

 offers impassable barriers. The action of the elements, the 

 attacks of insects and larger animals, the exhaustion of the soil 

 by other vegetables, the constant lengthening of the roots, 

 making the circulation too extensive, are all causes constantly 

 operating to prevent the duration of the most of forest, trees 

 beyond one or two centuries. But there are cases in which 

 favorable circumstances have conspired to lengthen out the 

 lives of particular individuals to as many thousands. Some 

 trees of great age have become subjects of history. The cele- 

 brated Chestnut of Mount Etna has a circumference of 160 

 feet. This tree has been said to be several trees united ; but 

 more recent and accurate observations have proved it to have 

 but a single root, and of course it is a single tree. Its age, by 

 any calculation, must be that of many centuries. Some of the 

 oldest Cedars of Lebanon are supposed to have an age of three 



191. What is the opposing theory? — 192. What of the duration of veg- 

 etables ? What is each year's growth of a d cotyledonous tree? What 

 limits its duration ? How old are some trees ? 



