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TEXTBOOK OF BOTANY 



pith-like parenchyma. As the young stem increases in age cambium 

 usually develops from the parenchyma cells between the bundles and 

 becomes a complete cylinder in the stem (Fig. 107). 



Fig. 106. A, photomicrograph of a cross section of a stem of moonseed vine; B, 

 one of the vascular bundles, adjacent pericycle fibers, and cortex greatly enlarged 

 to show the small cambium cells in contrast to the enlarged and diflFerentiated cells 

 of the xylem and phloem. The outermost phloem cells have been crushed by the 

 enlargement of the inner younger cells formed by the division of cambial cells. 



Since the xylem of woody perennials increases in diameter from year 

 to year, it presses against the encircling bark. Some of the cells of the 

 bark become crushed, and the outer dead parts of the bark are ruptured. 

 As a result, the outer bark of woody perennials is usually furrowed and 

 irregularly broken. 



In most parts of the world cambial growth is limited to a few months 

 of each year. In woody perennials of temperate climates it begins in the 

 young twigs about the time the buds begin to open in spring, and some- 

 what later in the older branches and trunk of the tree. Two or three 

 months later the cambium becomes dormant in the trunk of the tree and 

 remains in that condition until the following spring. Each season cambial 

 growth begins first in the twigs and continues longest in them. Growth 

 of the cambium appears to be dependent upon hormones from the 

 leaves, but the evidence upon which this inference is based is too 

 limited for final conclusions at present. 



The xylem cells and vessels that develop in spring are usually larger 

 than those that develop toward the close of the growth period in summer. 



