SUNFLOWER STEM 63 



Within these tissues of the cortex (a general term 

 including the tissues described under the headings 2 

 and 3) lie 



4. The vascular bundles, which are wedge-shaped, 

 and are arranged in a ring : according to the stage of 

 development of the stem, and the point at which the 

 section is ta,ken, the bundles may be more or less 

 completely joined laterally with one another. In old 

 stems, and at or near the nodes, this lateral fusion is 

 most complete : still, under any circumstances, the 

 originally separate bundles can easily be recognized. 



5. Centrally, i.e. within the ring of vascular bundles, 

 is the parenchymatous pith, consisting of thin-walled 

 cells, which have for the most part lost their activity, 

 having no protoplasmic contents, and are filled with 

 air : hence the whiteness of the fresh pith. In material 

 which has been a long time in spirit, the air may have 

 been removed by the alcohol, but this is usually a slow 

 process. 



II. Choose out the thinnest of the sections, and 

 examine it with a higher power, starting as before 

 from the periphery of the stem. 



1. The epidermal layer will be seen to consist of 

 cells contiguous with one another, without intercellular 

 spaces, excepting occasional stomata : the structure of 

 these will be studied in detail in specimens where they 

 are more numerous. The walls, and especially the ex- 

 ternal and internal walls, are thick, highly refractive, 

 and show a stratified structure. In chlor-zinc-iodine 

 they are blue (cellulose : see p. 37) with the exception 

 of the outermost layer the cuticle : this is a con- 

 tinuous well defined layer, which stains yellow, and 



