WOOD AND BARK. 295 



Fig. 25 shows the section as arranged for the experiment of determining 

 whether the new layer of wood would be developed from the 

 old wood or from the bark. 



Fig. 26 represents the appearance of the new wood (b) which was deposited 

 upon the metal, (a) after the removal of the bark in September. 



Fig. 27 gives the microscopic structure of a horizontal section of the 

 elm wood and bark directly over the metal. Next to the tin 

 was a thin layer of parenchyma, (a) connected to the inner layer 

 of bark by medullary rays, (c) which were as numerous as in 

 the other parts of the new wood, and passed directly from the 

 bark to the metal, whether examined in a horizontal or verti- 

 cal section. The cork cells, (f) bast (d) and parenchyma (e) 

 of the bark, and the woody fibre, (b) ducts (g) and medullary 

 rays of the stem, are clearly visible in this section. 



Fig. 28 is a view of the longitudinal section of the branch of an apple 

 tree which was girdled in May, 1870. After growing four 

 years and bearing fruit, it was cut in 1874. There were then 

 many dead twigs upon it, and it was evidently in declining 

 health. The section shows how the sap-wood was becoming 

 dry, and changing into heart wood, so that the channel for 

 the transmission of the sap from the roots to the leaves was 

 almost closed. The girdling was complete, so that the elabo- 

 rated sap from the leaves could not descend below it. 

 A is the top of the nearly horizontal branch. 



Fig. 29 shows how a branch of a wild grape vine, after being girdled, 

 formed new wood from both above and below, and thus made 

 a new passage for the downward flow of the sap. The wood 

 developed from beneath the girdle was formed from sap 

 elaborated in other branches. 



Fig. 30 is a section of the stem of a young bass tree, which shows that 

 when there is no foliage below a place girdled early in the 

 season, there can be no deposition of new wood, while it may 

 be as abundant as usual above the girdle. 



Fig. 31 represents a section of a stem of a young red maple, girdled June 

 twenty-third, which is enlarged above the girdle, but not below. 



Fig. 32 exhibits a similar section, girdled July twenty-first, upon which 

 was produced a new growth of both wood and bark, which 

 resulted from the fact that the cambium layer was so far 

 organized by midsummer as to furnish a conducting medium 

 for the elaborated sap. 



Fig. 33 shows the microscopic structure of the ordinary bark of a young 

 red maple. 



A, periderm or cork. 



B, primary parenchyma. 



C, secondary parenchyma. 



D, bast fibres. 



E, woody fibre of trunk. 



F, vessels or ducts in wood. 



G, medullary rays connecting bark and wood. 

 H, recent parenchyma of inner bark. 



