122 



A TEXTBOOK OP 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 l)undles, called xylem, contain the ducts, and 

 conduct water thrpugh the stems. Inside the cyhnder of 

 bundles is the very cUstinct 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 



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 fa.st lo.sing its chloro- 

 phyll, and known henceforth b}^ its morphological name of 

 CORTEX, while the temporarj^ coUenchyina and epidermis are 

 being replaced tjy layers of waterproof cork, made bj- a cork 

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

 of these features can bo 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 \vithin 



