INFLUENCE OP LIGHT UPON DIRECTION OP STEM. 199 



tied round its summit, and has been kept so for a long period. 

 The plant would then push up that one of the side-shoots, which 

 is most nearly in the line of the trunk ; and this, increasing in 

 length and diameter in successive years, will gradually present 

 more and more the appearance of a continuation of the lower 

 part of the stem ; whilst that which was bent down, presents 

 the aspect of a branch. 



311. The same means is adopted, to repair a natural injury. 

 Thus, the upper half of a large elm tree, which had a long 

 straight trunk, the lower half being without branches, was broken 

 off by a violent gust of wind. From the complete absence of 

 leaves in the trunk that remained, it was not expected to survive ; 

 but, being in full sap at the time, the abundant nourishment it 

 contained, occasioned the development of buds which were pre- 

 viously inactive; and a great number of small branches soon 

 issued from the stump. Of these, the upper ones have grown 

 most rapidly ; and the two highest, which were at first nearly 

 horizontal, have gradually changed their direction, so as to follow 

 the line of the upright stem ; and it now seems as if the trunk 

 had originally divided, at that point, into two minor ones, so 

 completely has all appearance of the accident been lost. 



3 1 2. The leaves of all plants have a very limited term of 

 existence. In temperate climates, most trees shed them during 

 the autumn, and pass the winter in a state of complete inactivity. 

 Before they fall off, the leaves usually change colour, some- 

 times very decidedly, as do those of the Beech ; and it has been 

 ascertained that at this period they absorb more oxygen, and give 

 out more carbonic acid, which indicates their commencing decay. 

 This absorption of oxygen has been shown, by experiment, to be 

 the immediate cause of the change of colour ; since the green 

 matter of leaves, when acted on by substances which readily 

 yield oxygen, is found to exhibit it. The separation of the leaf 

 from the stem, is probably due to several causes. During the 

 latter part of the summer, some of the vessels become choked 

 up with solid matter ; and those which proceed from the interior 

 of the trunk, are overstrained by the addition to its diameter 

 which has taken place, so that they are easily ruptured. The 



