708 ECOLOGY 



Cork is relatively impermeable to air as well as to water, and after its formation, 

 carbohydrate synthesis and aeration in subjacent tissues become greatly reduced, 

 except in the neighborhood of lenticels. Cork is also a poor conductor of heat, so 

 that changes of temperature are slower within the plant than outside. Cork pre- 

 vents the invasion of living tissues by parasitic fungi and bacteria; a freshly cut 

 surface of a potato tuber develops wound cork so rapidly that after twelve hours 

 bacterial infection is impossible. Cork cells, like tracheids, tracheae, and bast 

 fibers, are more efficient dead than living. Such local growths of cork as the wings 

 of the winged elm and the warty projection of the hackberry are probably of no 

 advantage to the trees producing them. Sometimes at the close of the vegetative 

 season, bark is not perfectly " ripened," that is, it contains considerable water, and 

 the various protective and mechanical elements are not fully formed. In many 

 plants transferred from warmer to colder climates the shoots die back in the winter, 

 because the vegetative season is too short to permit the ripening of the bark and 

 wood. 



Various bark features. Color. Young bark commonly is green, because the 

 cortical chlorophyll is evident through the transparent epidermis. Soon the stem 

 ceases to appear green, the chief cause of change in color being the development of 

 the cork layer, whose opacity makes the chlorophyll invisible. The common >ark 

 colors are gray, brown, and black, but red occurs, as in some dogwoods, and white, 

 as in some birches. As the tree matures, the characteristic bark color may be seen 

 only on the younger branches, if the older limbs are furrowed. In a few cases, is in 

 the mistletoe, moonseed, sassafras and greenbrier, the relative freedom from cork 

 formation permits the green color to remain evident longer than usual. Such 

 green-stemmed trees as the bamboo and the banana are in reality gigantic herbs, in 

 which ordinary bark does not develop. Often the exterior and the interior of the 

 bark are differently colored, as in the hemlock, where it is black without and red 

 within, and as in the yellow-barked oak, which is named from its inner bark, the 

 outer bark giving rise similarly to the name, black oak. Bark colors, especially 

 interior colors, often are due to the presence of various excreted products, such as 

 tannins. Advantages in the various colors are not to be looked for. 



Smoothness and roughness ; exfoliation. While the epidermis persists, young 

 stems are smooth, except in the neighborhood of lenticels and leaf scars ; the latter 

 are of various shapes and sizes, and differ widely in the number and arrangement 

 of the vascular strands, whose severed and healed surfaces are conspicuous as s light 

 emergences within the scar (fig 1059). For a few years most stems remain sn ooth 

 or smoothish, owing to the development of bark tissues as the stem increases in 

 diameter. In some trees (as in the beech) continued lateral growth cause; the 

 bark to remain thin and smooth throughout life; the tropical rain forest in particu- 

 lar is rich in smooth-barked trees. In various palms the bark is soft and spongy, 

 hence affording an excellent habitat for epiphytes. In most trees new phellogen 

 areas develop at deeper levels or lateral growth fails to keep pace with diametral 

 increase, so that the bark splits and becomes variously roughened. Some trees, 

 as' the bur oak, become furrowed very early, while others, as the basswood, remain 

 smooth-barked for a long time, but ultimately become furrowed. " Alligator " bark 

 is caused by the division of the bark into blocks by somewhat equidistant trans- 

 verse and longitudinal furrows (as in Nyssa). 



