6 Practical Plant Biology. 



very slightly nearer or further are out of focus and appear blurred. 

 Hence when we look at a microscopic field with a high power 

 only those objects which are at a certain level in the preparation 

 are distinctly visible to us, and in order to appreciate the relation 

 of the various objects in the preparation to one another we must 

 continually keep shifting the focus up and down. Thus while 

 using the high power it is advisable to keep one's finger on the 

 fine adjustment and to move its milled head continually back- 

 wards and forwards. 



While moving the focus for the high power it becomes ap- 

 parent that each cell is bounded by a transparent membrane the 

 cell-wall. As we look down on the cell its outline is generally 

 hexagonal or pentagonal. One can see that the wall is colourless 

 and that the light from the mirror passes through it. It is trans- 

 parent. When the focus is at* a certain level the thickness of the 

 wall is seen in section. By carefully focussing upwards with the 

 fine adjustment one can see that the wall is continuous across the 

 top of the cell next the cover-glass, and with skilful manipulation 

 of the mirror and diaphragm one can see that the outer surface 

 is finely corrugated. In the same way the cell is closed below. 

 The cell, in fact, at which we are looking is a flat hexagonal or 

 pentagonal box. The walls of this box are made of a transparent 

 material. Looking through the transparent top we can see the 

 contents of the box illuminated by the light transmitted through 

 the equally transparent bottom. 



The most easily observed object within this cell is the nucleus. 

 Its position in the cell appears to be fortuitous. It is dark or 

 bright according as the diaphragm is closed or open. It is 

 colourless. At first sight it appears to be disc-shaped, but when 

 very careful focussing is used it is possible to see that it is ap- 

 proximately globular in form ; it appears disc-shaped because 

 only one level or section of it appears in focus at one time. 

 By focussing deeper and deeper one can compare a series of 

 these focal levels, or optical sections, with one another and 

 mentally reconstruct from them the shape of the whole. It is 

 evident that the optical sections of an approximately spherical 

 object are a series of more or less circular discs. The nucleus 

 is then a spherical or globular body. 



During these observations another substance within the cell 

 will be forcing itself on our attention. Round the nucleus and 

 adhering to the inside of the cell-wall is a colourless and slime- 

 like material. When the diaphragm is closed this material is 

 lighter than the nucleus, when the diaphragm is open it is less 



