78 Proceedings of the Ohio State Academy of Science 



at the end of each growing season by the formation of a cleavage 

 plane or separation layer through the base of the petiole. They 

 also prepare for winter by developing elaborate winter buds. 

 The function of the winter buds is mainly to check evaporation 

 from the delicate stem tips during the periods of freezing and 

 thawing. 



Many of the smaller branches and twigs of a tree especially 

 when growing in a dense forest are continually dying off. But 

 the tree rids itself of these dead branches by forming a collar of 

 tissue from the cambium layer around the base of the dead 

 branch, which finally covers over the wound when the dead mem- 

 ber falls off. This process is known as natural pruning. By the 

 formation of a similar callus other wounds are covered up. 

 There is still a more remarkable process present in many trees 

 by which surplus living branches are cut off in one way or an- 

 other. Terminal and lateral buds are also commonly cut off. 

 This process is known as self-pruning. The most common 

 method is by the formation of a cleavage plane in a basal joint 

 or in the annual nodes of growth. In some genera brittle zones 

 are produced. The self-pruning process is very highly devel- 

 oped in the cottonwood, white oak, white elm, and silver maple. 



Trees grow in height only at the tips of the main stem or 

 branches. Some trees are naturally shortlived ; others attain an 

 enormous size and age, but from the very nature of their up- 

 right development their life must sooner or later come to an end. 

 In some cases the individual organism may continue by a new 

 development from sprouts growing out of the stump or the roots. 



All of our trees bear flowers and seeds. After arriving at 

 a certain age depending on the species, the tips of some twigs or 

 the axillary buds will develop flowers. In the more highly de- 

 veloped and typical flowers four sets of organs are present ; the 

 calyx composed of sepals, the corolla composed of petals, the 

 androecium composed of stamens, and the gynoecium composed 

 of carpels. The two essential sets of organs in the flower are 

 the stamens and the carpels. These may both be in the same 

 flower, when the flower is said to be bisporangiate or in separate 

 flowers, when the flower is monosporangiate. If the two kinds 

 of flowers are on one individual the plant is monoecious, if on 



