TOP-WORKING SEEDLING PECAN TREES. 13 



structure of cambium tissue gives us a clearer conception of its extreme 

 delicacy. It is one of the most sensitive and delicate substances in all 

 nature. Exposure to the air will kill it and completely destroy its func- 

 tions in a few seconds. It is easily crushed by slight pressure and 

 quickly killed by exposure to drying, frost, moisture, and sunlight. 

 Nature shows her extreme care of it, for in making bark she has formed 

 for the delicate cambium a perfect protective covering. Like the cam- 

 bium, the bark is composed of cells, as in fact are all animal and vege- 

 table structures. But the cells of the bark have thick walls of a tough, 

 corky substance, and each cell contains air instead of protoplasm. The 

 corkiness of the bark makes it an impervious, waterproof covering that 

 does not allow the cambium to be dried out or to be washed by external 

 moisture. The air in the bark cells being in a still condition, is a non- 

 conductor of heat, and layer of bark overlapping layer, the cambium is 

 completely covered with a dead-air blanket. This keeps it from being 

 frozen in winter and from being overheated in summer, just as a dead- 

 air space in the walls of a building protects from extremes of heat and 

 cold. From this it is plain that nature takes great pains to cover and 

 protect the delicate cambium from all external influences. This stands 

 in striking contrast to the careless manner in which many propagators 

 and planters handle the delicate parts of trees. It also explains why 

 some budders get such a small percentage of living buds and planters so 

 few living trees. 



Cambium is the building material of plants, and without it growth is 

 impossible. It covers every portion of the tree from the topmost terminal 

 bud to the deepest root tip like a living blanket. During the growing 

 season the cambium cells divide lengthwise, forming new cells. These 

 divide again and grow, and new cells are formed, until by fall a com- 

 plete mantle of bark covers the outer surface of the cambium, while 

 within it has built up a solid layer of the woody structure of the tree. 

 A few rows of cambium cells are left in an embryonic condition to carry 

 on growth the following year. The cambium is thus the only tissue of 

 the tree that retains from year to year the power of active growth. The 

 layers of wood and bark, after performing their functions for a few 

 seasons, gradually die and are overlapped by new layers; but the cam- 

 bium remains living throughout the entire life of the tree, even if it be, 

 as in the giant Redwoods, thousands of years. 



