SUNFLOWER STEM 37 



large multicellular hairs, which may be recognized as being 

 products of the epidermis. 



2. Beneath this single epidermal layer lies a band of tissue, 

 several layers of cells in width, the walls of which are thickened 

 at the angles where three or more cells meet, the cell-cavity 

 being thus made oval or circular in transverse section ; this is 

 the chief characteristic of collencbyma, of which this is a good 

 type. Below this lies 



3. A band of thin-walled parenchyma, in which are dotted 

 here and there resin-passages. 



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 taken, 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 proto- 

 plasmic 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. 



i. The epidermal layer will be seen to consist of cells con- 

 tiguous with one another, without intercellular spaces, but 

 with occasional stomata : the structure of these will be studied 

 in detail in specimens where they are more numerous. The 

 walls, and especially the external and internal walls, are thick, 

 highly refractive, and show a stratified structure. In chlor- 

 zinc-iodine they show the characteristic blue of cellulose 

 with the exception of the outermost layer the cuticle : this 

 is a continuous well-defined layer, which stains yellow, and 



