LECTURE II. 



FROM the few observations already made some idea may be 

 formed of the different parts of the cells taken from the leaf-skin. 

 It is difficult, however, to realise immediately the physical nature 

 of these different microscopic objects. All tissues of plants are 

 made up of cells resembling, more or less closely, those we have 

 examined in the skin of the leaf of Tradescantia virginiana, so 

 that if we can obtain clear ideas as to the nature of the parts of 

 these cells we can apply them with little modification to all others. 



It is easy to see the nucleus and the rest of the protoplasm 

 when looking through the walls of the cells taken from the skin 

 of the leaf. The substance forming the wall is therefore evidently 

 transparent ; and it is also without colour. One can see, how- 

 ever, the surface of the cell-wall and it is also quite easy to dis- 

 tinguish it from the surrounding water. We distinguish it just 

 as we distinguish a piece of glass in air or in water, not neces- 

 sarily by its different colour, but because it bends the rays of light 

 differently from the surrounding air or water. It has, in fact, a 

 different refractive index. The substance forming the cell-wall 

 is then transparent, colourless and has a higher refractive index 

 than water. It is owing to these physical properties that it looks 

 bright when illuminated with an open diaphragm and dark when 

 the diaphragm is closed. 



The torn edge of the preparation is formed by partially empty 

 cell-walls. By pressing on the cover-glass with a needle it is easy 

 to distort these pieces of cell-wall. When the pressure is relieved 

 they recover their former shape. The cell- wall evidently possesses 

 a certain amount of rigidity and is elastic. It also possesses 

 tenacity and is tough. This will be realised when one remembers 

 that a sensible pull had to be exerted in tearing the piece of skin 

 and that the strip torn off from it is tough and can sustain a con- 

 siderable pull. It is also to be remembered that the actual cross 

 section of cell-wall sustaining this pull is very small ; for but a 

 very small portion of the strip is composed of the walls. The 

 actual strength of the cell-wall compared with other substances 



