92 COMPENDIUM OF GENERAL B01ANY. 



the cell- walls) ; they may be scalariform, reticular, or annular. Tra- 

 cheids have border ed-porous slanting end -surf aces ; this is because 

 the originally transverse septa in the stem incline to the right or 

 left. Fig. 56, which represents a longitudinal radial section of a 

 dictyoledonous stem one year old (CEnotherd) illustrates the im- 

 portant difference between vessels which have grown in length with 

 the other tissues and those which began development at the close of 

 the vegetative period. Similar anatomical relations also exist in 

 the innermost annual rings of the dicotyledonous stems. 



A tangential longitudinal section again shows the characteristic 

 appearances of the medullary rays. They appear as two-edged cell- 

 rows or cell-groups, or as bands of cells of one or more layers in 

 thickness, when the ends are invisible. The typical appearance of 

 medullary rays in both longitudinal sections (radial and tangential) 

 is as marked in the secondary cortex as it is in the wood. 



The difference between splint-wood and heart-wood in dicotyle- 

 dons and conifers has long been recognized. The variously colored 

 woods of commerce always represent the heart- wood of the trees 

 from which the wood has been taken, that is, the inner annual 

 rings, which are usually very numerous. Splint- wood is from the 

 younger tissue, therefore from the outer layers. The heart- wood is 

 of a darker color than that of the splint, due to a coloring substance 

 deposited in the cell- wall. Besides the coloring materials, tannin 

 also is found in the cell-walls of the heart- wood. The lumina of the 

 vessels and tracheids are either closed by a hardened gum ( " heart- 

 gum "), more rarely by a resin, or by cells (tyloses), which latter 

 grow through the pores from the immediate vicinity. These 

 changes render the heart-wood functionless as far as the conduction 

 of water and the storing of carbohydrates is concerned, but it is on 

 the other hand less subject to decay. The latter property and 

 especially the occlusion of the vessels, makes such heart- wood forma- 

 tions especially important, because they serve as a protective coat- 

 ing not only to the inner portions of the stem, but also to external 

 injuries; for example, on the cut or broken surfaces of stems and 

 branches there is formed a resinous or gummy protective covering ; 

 hence the name ' ' protective gum " or " wound gum ' ' (callus) 

 (FRANK). 



I may only mention the interesting c ' overflow ' ' growths which 

 proceed from the cambium in the case of injuries. Literally speak- 



