•^ h'lEi.n Mantai. 



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 espe- 

 cially when growing in a dense forest are continually dying 

 off. Rut the tree rids itself of these dead branches by form- 

 ing a collar of tissue from the cambium layer around the base 

 of the dead branch, ^j'hich finally covers over the wound 

 when the dead member 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 m^ore remarkable 

 process present in many trees by which surplus living 

 branches are cut off in one way or another. Terminal and 

 lateral buds are also commonly cut off. This process of 

 abscission 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 developed 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 upright development their life must sooner or later come 

 to an end. In some cases the individual organism may con- 

 tinue 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 dependng on the species, the tips of some 

 twigs or the axillary buds will develop flowers. In the more 

 highly developed and typical flowers four sets of organs are 

 present; the calyx composed of sepals, the corolla composed 

 of petals, the andrecium composed of stamens, and the gyne- 

 cium composed of carpels. The two essential sets of organs 

 in the flower are the stamens and the carpels. These may 

 i)Oth be in the same flower, when the flower is said to be 

 bisporangiate or in separate flowers, when the flower is mono- 

 sporangiate. If the staminate and carpellate flowers are on 

 one individual the plant is monecious, if on two distinct indi- 



