Tree tissue. It protects the trunk against the elements, disease, animal attack and fire. It is perforated by a large number of fine breathing pores called lenticels, through which oxygen diffuses. Bark is continually replaced by a living layer of cells called the cork cambium or phellogen. The London plane ("Platanus × acerifolia") periodically sheds its bark in large flakes. Similarly, the bark of the silver birch ("Betula pendula") peels off in strips. As the tree's girth expands, newer layers of bark are larger in circumference, and the older layers develop fissures in many species. In some trees such as the
Northern celestial hemisphere referred to as the Northern Hemisphere. For the purpose of celestial mapping, astronomers may conceive the sky as the inside of a sphere divided into two halves by the celestial equator. The Northern Sky or Northern Hemisphere is therefore the half of the celestial sphere that is north of the celestial equator. Even if this geocentric model is the ideal projection of the terrestrial equator onto the imaginary celestial sphere, the northern and southern celestial hemispheres is not to be confused with descriptions of the terrestrial hemispheres of Earth itself. Northern celestial hemisphere The northern celestial hemisphere, also called the