AN INTRODUCTION TO PLANT BIOLOGY 71 



when the stems of trees are growing rapidly and this layer is soft, 

 boys make whistles at the end of willow sticks by pounding the bark 

 until it is loosened from the wood so that as a ring it can be slipped 

 off, and then certain notches are cut in the wood. Also, the 

 same soft substance between the bark and wood of one species of 

 elm trees is the delicious "slippery-elm bark," which in a dry and 

 powdered form is sold by druggists for alleviating irritations of the 

 throat. 



In the center of the soft substance between bark and wood of 

 such stems is a layer of cells which are active in growth and division. 

 This is the growing layer or cambium. On its inner side new wood 

 cells are formed and added to the older wood of the stem, and on its 

 outer side the new cells formed are added to the inner surface of 

 the bark. Hence growth and division of cambium cells adds new 

 cells to both wood and bark, resulting in increased diameter of the 

 stem. 



On the outside of the rind of the bean stem is a thin layer of 

 cells, the epidermis. Some cells of the rind are green in color, which 

 is due to a substance common in leaves and known as leaf -green or 

 chlorophyll. 



With a sharp knife or section-razor, make a cross cut (transverse 

 section) of the stem. Examine with hand-lens, and note relative 

 thickness of rind and wood. In the center of the wood is a softer 

 substance, known as pith; and in the oldest parts of the bean stem 

 there is a cavity in the pith. 



Split open lengthwise (longitudinal section) a stem from an old 

 bean plant, and note the extent of the pith and of the central or 

 pith-cavity. Tear the wood apart lengthwise, and note that it is 

 "stringy." Pay special attention to the arrangement of the wood, 

 bark, and pith in a longitudinal section at a branch. Make dia- 

 grams showing position of rind, cambium, wood, and pith. 



70. Microscopic Study of Bean Stem. (D) With a razor or very 

 sharp knife cut a very thin transverse slice or section of the stem, 

 mount in water on a glass object-slide. Examine with low power 

 first. Examine the rind and its outermost layer (epidermis), the 

 wood, and the pith. These are examples of the tissues or building 

 materials of the plant. Use a higher power of the microscope, and 

 note that each of these tissues resembles a piece of honeycomb or 

 hollow blocks set together like bricks in a wall. These are the plant 

 cells. Some of the substance in the cells is the living matter or 

 protoplasm, but some cells in the central part of the stem appear to 

 be empty. The fact is that such empty cells are dead, and only their 



