60 SECONDARY INCREASE IN THICKNESS 



ILLUSTRATIVE STUDIES 



1. Make cross and longitudinal sections of a stem of Aris- 

 tolochia that is several years old (Fig. 24). The sectioning will 

 need to be done on a sliding microtome, and the sections should 

 be stained with erythrosin and iodine green, or with safranin and 

 haematoxylin, and made into permanent mounts in balsam. 

 Study with low and high powers and note the changes which each 

 tissue has undergone since the last condition studied. What 

 tissues have increased? decreased? been broken? crushed? 

 Have new tissues appeared? Make drawings and diagrams to 

 show the changes discovered. 



2. Pay particular attention to the new medullary rays. 

 What evidence can you find as to how they have originated? 

 Assuming that the function of the rays is to carry water and food 

 radially and to store them also, can you see special need of the 

 new rays ? 



3. Find the primary xylem and phloem (page 36) of the vas- 

 cular bundles. Have they been changed in position and condi- 

 tion since first they were formed from the procambium ? 



4. Study cross sections of cornstalk close to the growing apex 

 and farther down where growth in diameter is well-marked. 

 As the stem increases in diameter, do the vascular bundles enlarge ? 

 Does the number of cells in them increase? Does the number 

 of vascular bundles increase ? What changes take place in the 

 ground tissue as stem enlargement progresses? Do you find 

 cambium in the vascular bundles? Draw a vascular bundle 

 on a large scale and show how it differs from a dicotyledonous 

 bundle (of Aristolochia, for instance). 



Good material for this study of corn can be obtained from 

 stalks of field corn about two feet high. By stripping away the 

 leaves and leaf sheaths the stem will be found extending about 

 six inches up from the ground with nodes and internodes in all 

 stages of development. 



