6 PRACTICAL BOTANY. 



as spores, pollen-grains, &c. This is effected by means 

 of gum. A thick layer of strong clean gum is laid on 

 the flat surface of a piece of pith ; this is allowed to 

 become nearly dry ; and then the pollen grains or spores 

 are dusted on to it ; these are then covered with an- 

 other thick layer of gum, and the whole is allowed to 

 dry. Sections are now made of the dried gum, and, on 

 their being placed in water, the gum is dissolved, and 

 the sections of the pollen-grains or spores are set free. 



Staining. It is often useful to stain sections in 

 order to bring out certain points in their structure, 

 which are difficult to observe under ordinary circum- 

 stances. A great number of colouring matters have 

 been used for this purpose, among which may be men- 

 tioned as the most useful: Hsematoxylin, Carmine, 

 Cochineal, Gold Chloride, various preparations of 

 Aniline, such as Safranin, Nigrosin, Fuchsin, Methyl- 

 green, Eosin, and Methyl-violet. 



Staining is best performed by placing a few drops of 

 the staining-fluid in a watch-glass and immersing the 

 sections in it. The exact strength of the fluid, and the 

 time of exposure of the sections to its action varies in 

 each case, and must be ascertained by preliminary 

 trials. As a rule, when differentiated staining is desired, 

 the best results are obtained by using a dilute solution, 

 and by exposing the sections for a long time to its 

 action. 



Haematoxylin. A number of preparations of this 

 colouring-matter are in use ; of these the following are 

 those generally employed for vegetable tissues. 



1. Alum solution of Hrematoxylin. Dissolve 0*35 grammes of 

 hsematoxylin in 10 c.c. of water, and add to it a few drops of a 



