4 o6 GENERAL BIOLOGY 



muscles through use, acclimation, immunisation; these and 

 many others are every day illustrations of the response of 

 the individual to -conditions of environment. Figure 237 

 shows the record in wood of a series of successive responses 

 on the part of the bough of an elm tree during the 25 years 

 of its life. The five dark rings in the center represent the 

 first five years of erect growth (1878-1882), while it was still 

 near the top of the tree, and abundantlyand symmetrically 

 lighted. It started in 1878 from a bud formed on the west 

 side of the top shoot of a three year old sapling. The 

 twelve close set rings following represent the scanty growth 

 of the next twelve years (1883-1894), during which it was 

 struggling for light beneath the higher branches that had 

 overtopped it. The larger growth ring for 1888 represents 

 the result of a windy season, when the tossing about of the 

 upper branches allowed this one to get more light. During 

 this time the bough was leaning slightly to westward, as 

 indicated by the greater thickness of the rings on that side 

 the lower side in the figure. The ensuing sudden unilateral 

 enlargement of the rings was due to an accident. Some 

 children climbing in the tree bent this bough down, and left 

 it in a somewhat drooping position. Thus, it was brought 

 out from the shadow into the light again, and the rapid 

 growth that followed was, in consequence of its position, on 

 the under side of the bough at the bend where this section 

 was made. It will be observed that for four years (1895- 

 1898) the addition of woody tissue was bilaterally sym- 

 metrical upon the lower side. Then another accident 

 changed the stress upon it and caused it to grow obliquely. 

 It chanced to overhang a walk, and in the spring of 1899 to 

 correct its drooping it was hung up lightly on a wire at- 

 attached to a fork above and a little to one side. The pull 

 <vas to the northward during the ensuing four years (1899 

 1902), and this, assisted by prevailing south-west winds. 



