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A TEXTBOOK OF BOTANY 



[Ch. IV, 2 



tissue between them is reduced to fine radiating lines, which 

 henceforth are called the medullary rays. These woody 

 parts of the bundles, called xylem, contain the ducts, and 

 conduct water through the stems. Inside the cylinder of 

 bundles is the very distinct pith. In the pith is much 

 starch, which is food for the next season's growth, though 

 it occurs also in medullary rays and bark, often in strikingly 

 symmetrical patterns when set forth in blue by the iodine 

 test. Outside of the cambium can be seen, though only 



I! 



Fig. 79. — Stages in the healing of a pruned stem, cl indicates callus, a 

 tissue which precedes the overgrowing bark. (After Curtis, from Duggar, 

 Plant Physiology.) 



imperfectly by hand lenses, the outer, or phloem, parts of 

 the bundles, which contain the sieve tubes and conduct food 

 through the stem. The remainder of the bark is composed 

 mostly of the former chlorenchyma, now fast losing its chloro- 

 phyll, and known henceforth by its morphological name of 

 cortex, while the temporary collenchyma and epidermis are 

 being replaced by layers of waterproof cork, made by a cork 

 cambium, and pierced here and there by the lenticels. AH 

 of these features can be traced very easily in nearly all twigs. 

 The tissues of plants have a remarkable power of healing 

 injuries which befall them. Any break in the soft tissues is 

 healed partially within a few hours, and completely within 



