50 



VEGETABLE HISTOLOGY. 



living cells, but the difficulty can be overcome by killing the 

 protoplasm or staining it. 



Raise the cover glass, add a drop of iodine stain and allow a 

 few moments for it to penetrate the cells, then cover and exam- 

 ine again. 



The cell-walls, scarcely stained, are distinctly visible. In 

 mature cells aggregated to form tissues, the common cell-wall 

 between two cells is made up of two like portions separated by 

 a layer of a slightly different chemical substance, which is 

 more soluble in reagents than the rest of the wall and shows 

 different reactions with test reagents and is known as the 

 middle lamella. Next to the cell-wall is a layer of protoplasm, 

 granular and deeply stained yellowish brown, called the prim- 

 ordial utricle. Somewhere within the cell will be seen a dense 

 body, the nucleus, surrounded by protoplasm and connected by 

 strings of protoplasm with the utricle. Between the strings 



B 



Fig. 33. Cells of Onion epidermis. A, surface view, 

 vacuole ; b, space where protoplasm has shrunken 

 utricle. B, cross-section, a, cuticle ; b, cellulose portion of cell-wall. 



. *>. Vjeus OI i.mioii epmermis. A, Bunace view, a., nucleus ; C, Cell-wall ; d, 



vacuole ; b, space where protoplasm has shrunken from cell-wall ; e, primordial 



are vacuoles, clear spaces filled with cell-sap. The nucleus 

 contains several smaller bodies, which are little nuclei or 

 nucleoli (plural of nucleolus). In some cells the protoplasm 

 may have shrunken away from the cell-wall at one end, leaving 

 a clear, apparently empty space. The cells dovetail into one 

 another, leaving no intercellular spaces, but forming a close 

 impervious layer. 



Like epidermal cells in general, the cells just described are 

 rather flat, but that is not apparent in surface view. This fact 

 is brought out in sections cut vertical to the surface of the 

 epidermis, which is best done by cutting through several scales 

 of the onion. The appearance is given in Fig. 33. The cells are 

 oblong in shape and the outer wall is somewhat thickened. 



Raise the cover glass from the section that was stained with 

 iodine, remove the excess of fluid and add a drop of sulphuric 

 acid (2 vols. cone, acid to 1 vol. water), after a moment replace 

 the cover glass and examine. The cell-walls are stained a deep 



