15 WAKE-ROBIN. 



3. The vascular bundle region. 



a. The irregular bundle-sheath surrounding the entire 

 region. 



b. Groups of conducting tissue distributed through the 

 region, the vascular bundles. Note their distribution 

 and approximate number. Each so-called bundle is 

 really a pair of strands, being composed of two inde- 

 pendent parts as follows: 



(1) The phloem strand, the outer part of the bundle, 

 consisting of angular, thin-walled cells of various 

 sizes. The larger cells are the sieve-cells through 

 which most of the manufactured foods are thought 

 to be transported. 



(2) The xylem strand, consisting of very thick-walled 

 cells of various sizes. In a young stem this tissue is 

 less abundant than the phloem, while in old stems 

 it is more abundant. It is closely associated with 

 the fibrous tissue that more or less invests the bun- 

 dle. In very young bundles between the phloem 

 and the xylem is a growing or meristem tissue known 



as the cambium. 1 



4. Draw a portion of the entire cross-section in which the 

 structural elements mentioned above are shown. 



IV. THE LEAF. 



Carefully peel off and mount epidermis from both surfaces 

 of the leaf and observe: 



1. The form of epidermal cells. 



2. The stomata. Each stoma consists of: 

 a. Two crescentic guard-cells, and 



1 In Monocotyledons all of this tissue, ceasing to divide sooner or 

 later, becomes changed into xylem or phloem, thus producing a so-called 

 " closed " bundle, while in Dicotyledons the persistent cambium makes 

 possible a constant increase in diameter, the bundle being therefore 

 ^open." 



