STRUCTURE OF STEMS 39 



tracheids, and wood parenchyma. The phloem is 

 similar to that of the corn stem. Outside of the ring 

 of vascular bundles is the pericycle, and outside of 

 that the cortex, both of the latter often becoming 

 changed into dead bark eventually. On the very 

 outside is the epidermis. Under the high power of 

 the microscope determine how the epidermis is pro- 

 tected against excessive evaporation. About how 

 many rows of cells are there in each of the tissues: 

 epidermis, cortex, and pericycle? 

 3. Is there any fundamental difference between the 

 young woody stem and an herbaceous stem? The 

 arrangement and structure of the bundles in the corn 

 stem are characteristic of the endogenous (mono- 

 cotyledonous) type of stem, and the arrangement 

 and structure of the bundles in the herbaceous and 

 woody stems are characteristic of the exogenous 

 (dicotyledonous) type of stem. 

 E. Secondary Growth of an Exogenous (Dicotyledonous) Stem: 



1. Examine a thin cross-section of an Aristolochia, 

 or other dicotyledonous stein, which is 3 or 4 years 

 old. A hand lens view of this is to be drawn, indi- 

 cating clearly the chief regions, pith, wood, and bark. 

 Do you find concentric rings in the wood? How 

 far apart are they and how many are there? These 

 are the "annual rings," which indicate the number 

 of periods of growth through which the stem has 

 passed. Each "ring" is, of course, the end (cross- 

 section) of a roughly cylindrical layer of tissue 

 formed during a given period of growth. 



2. Now study the cross-section under the microscope, 

 and make a V-shaped drawing of a portion of the 

 section, showing samples of each kind of tissue 

 in proper position. What characteristics mark 



