4 Practical Plant Biology. 



repeatedly changed and the effect of these changes observed ; the 

 aperture of the diaphragm should be altered and the changes in 

 appearance so caused studied. It will be found that while a closed 

 diaphragm causes the outline of a cell to appear dark, this outline 

 becomes brilliant when the diaphragm is opened. Similar changes 

 may be observed in the nucleus. By nearly closing the diaphragm 

 you will be further able to perceive that the cells are not absolutely 

 clear but are more or less shaded by the presence of a haziness 

 especially near their boundaries and round the nucleus. This 



-Tradescantia virginiana, lower epidermis, microphotograph x 100. 



hazy substance together with the nucleus is the protoplasm of the 

 cell. The use of the high power (or objective) will show the 

 parts of the cells more clearly. It is necessary to use great care 

 when changing from the low to the high objective. It is well 

 first to assure oneself that the cover-glass is quite clean and that 

 there is no water on its upper surface ; after this, carefully set 

 the cell selected for special examination exactly in the centre of 

 the field. With microscopes of the best construction it is only 

 necessary to turn the nose-piece round bringing the high power 



