The Structures and Processes of Stems 



125 



Fig. 73. Cross section of portion of a 

 rattan stem photographed through a 

 microscope. Note the ring of bundles 

 near the outside, with the heavy-wallcd 

 mechanical tissue and the scattered 

 bundles within. 



layer between the wood and 

 the bark, and the diameter is 

 increased by the addition of 

 successive layers of tissues 

 built by these cells. 



Every one who has made 

 willow or hickory whistles has 

 become acquainted with the 

 cambium. In early spring the 

 cambium cells are dividing 

 actively, and the cambium 

 layer can be broken by tap- 

 ping on the bark. The whole 

 bark can then be readily 

 stripped from the wood. 



The monocot stem. The monocot stem, like dicot and 

 conifer stems, is bounded externally by an epidermis which 

 closely resembles that of the leaf. The groundwork of the 

 stem is made up of parenchyma, which is commonly called 

 the pith. The parenchyma is usually composed of thin- 

 walled cells, and is the principal tissue for the temporary ac- 

 cumulation of foods ; from it the sugar solution is obtained 

 when the stems of sorghum and sugar cane (Fig. 48) are crushed. 

 In a monocot stem the bundles are scattered, instead of being 

 arranged in a cylinder as they are in a dicot stem. In the 

 hollow stems of grasses they are scattered through the cyl- 

 inder of parenchyma tissue ; in a cornstalk, a shoot of as- 

 paragus, or the trunk of a palm they are distributed through 

 the whole stem. As in the dicot and conifer bundles, the 

 water-conducting tissue is on the side next the center of the 

 stem, and the food-conducting tissue is on the side toward 



