CELLULAR ANATOMY OF THE STEM 223 



Pedagogics. - - Much as in the preceding. For both stems 

 it is best to use prepared and mounted sections, which show 

 with perfect clearness the cellular character of all the systems. 

 These should be given students, of course, only after they have 

 made out all possible with their own rougher hand-made ones. 

 Most of the work must be done with cross-sections, as the 

 longitudinal can be made to show very little unless cut ex- 

 tremely thin ; but students, after some instruction, soon learn 

 to recognize the tissues from a single view. 



In Corn, as matter of observation, the companion cells at 

 angles of the sieve tubes should not be missed. 



In Aristolochia, the sclerenchyma ring, continuous when 

 young, and broken by expansion of stem later (when the stem 

 twines and needs it no longer for support), should be noted. 

 This is a particularly easy stem for tracing the development 

 of the systems of tissues, which may be done by sections 

 at intervals along the stem. From the Aristolochia, through 

 young Oak twigs, the transition may be followed from 

 the distinct bundles of young stems to the woody mass 

 of older stems, in which the separate bundles are lost. 

 Note homologies of the parts of the young twigs and old 

 wood, particularly in the annual rings and the medullary 

 rays, which latter in the Oak form the shining plates sought 

 for in " quartered "' Oak. In Aristolochia, note morphology 

 of pith, and of medullary rays as simply in origin the paren- 

 chyma between the bundles. Also note usual identity of 

 starch-making system and cortex. Trace here homology 

 with the leaf, and how the green parenchyma of the leaf 

 answers to cortex. Note how, in the older stem, epidermis 

 is replaced by cork layers. 



Exercise 45 is most valuable ; it will impress the real rela- 

 tion of the two types of stems ; there will be a difficulty with 



