MICROSCOPIC OBJECTS. 3 



Fig. 5. Cells from the Apple (Pyrus malus), x 100. 



It will be noticed that some of these cells are darker than 

 the others ; these have been treated with sulphuric acid and 

 iodine, which causes the cell- contents or protoplasm to con- 

 tract towards the centre of the cell. Within each cell will 

 be found one or more nuclei, and within these the nucleoli ; 

 it is supposed by some that these are the life-centres of the 

 cell. 



Sulphuric acid and iodine colour the cellulose of the cell- 

 walls blue, and at the same time render the nitrogenous pro- 

 toplasm yellowish brown. Cells are often found with thickened 

 walls (see fig. 18) , and also with pits or dots, from the irregular 

 formation of the cellulose (see figs. 8, 10, 14, &c.). 



The cells are from -^ to T ^ inch in length, and from 

 to -^ inch in breadth. 



They are best mounted in liquid. 



Fig. 6. Transverse Section of the Pine-apple (the fruit of 

 Ananassa sativa) , x 240. 



This section forms an example of the ordinary loose paren- 

 chymatous tissue so commonly met with in various fruits 

 and other cellular growths, such as exists also in most 

 annual plants. It will be found, however, that in the case of 

 fruits the cells are usually large, while in the stems, leaves, 

 &c. of plants the cells are generally small. 



It is best preserved either in the dry state or in liquid. 



B 2 



