PLANTING AND OARE OF TREES 45 



cess may be completed without injury to the tree. Scars, more or 

 less entirely covered by callus, are familiar objects on trees that 

 have been pruned. They are shown in figures 44 and 47. The pro- 

 cess of grafting depends upon the growing together of this wound 

 tissue found on two different twigs when their cambium layers are 

 held in contact. 



Reproductiofi — A tree is a flowering plant. Except for 

 a few forms like our fruit trees, Magnolias, Horse-chestnut, Catalpa, 

 Locust and Tulip Tree the flowers are generally inconspicuous 

 and consequently often overlooked. From perfect flowers come 

 seed, and seeds are the natural means of reproduction in flowering 

 plants. Most species have both sexes upon the same individual, 

 with male and female organs in the same flower, or in separate 

 blossoms on the same tree. The Willows, Poplars, and Ashes, 

 however, have the different sexes on separate individuals, and are 

 called, therefore, dioecious. A single tree of a dioecious species 

 will bear flowers of only a single sex. Cuttings carry the sex of 

 the tree from which they were made. Forms like the Lombardy 

 Poplar which produce only male flowers never set seed and can be 

 multiplied, therefore, only by nonsexual methods such as by 

 cuttings. 



Trees do not bear with equal abundance every year. Thus, al- 

 though a few cones may be produced each season, the White Pine 

 in the northeastern states has a good seed year but once in about 

 six years. Eeproduction in relation to propagation is further dis- 

 cussed in the following chapter. 



