STAINING SECTIONS 261 



11). Fixed in Perenyi's fluid, a very useful fixing 

 agent for embryological work. Stained in Mayer's carm- 

 alum for a few minutes, washed in water to remove alum, 

 dehydrated and mounted in balsam. It is often advisable 

 to dilute the stain with water and leave the object in it 

 for a longer time. This is a particularly good stain for 

 Polyzoa, Hydrozoa, larval echinoderms, and other small 

 objects. 



Double Staining Sections. Sections of stems, roots, and 

 leaves are double stained to differentiate between those 

 cell- walls which consist of pure cellulose, and others that 

 have been modified by lignification. The method depends 

 on the selective affinity of some dyes for cellulose, and of 

 others for lignified tissues. 



Clematis vitalba. Transverse section of stem (Plate 17, 

 Figs. A, B, and C). Stained by immersion in a single 

 solution, picro-aniline blue. Cellulose walls become blue, 

 and lignified tissues yellow. 



Cycas revoluta. Transverse section of Petiole (Plate 

 6). Stained in Ehrlich's haematoxylin and counter- 

 stained in safranin. Haematoxylin stains cellulose but 

 not lignified tissues, the safranin stains the latter red. 

 Immerse the section in Ehrlich's hsematoxylin for a few 

 minutes, wash out excess of stain in 70% alcohol. Wash 

 in tap water, the bluish-purple colour is converted to 

 deep blue. Counterstain by immersion for a few minutes 

 in alcoholic solution of safranin, wash out excess of colour 

 in 95% alcohol, dehydrate in absolute alcohol ; clear in 

 xylene and mount in Canada balsam. 



Staining in Bulk and Counter staining Sections. Most 

 animal tissues, and such plant structures as root apices 

 and flower and leaf buds, are stained in bulk prior to 

 imbedding and cutting the sections. The sections after 

 fixing on the slide may be counterstained in a coal-tar 

 dye or picric acid. 



Dividing Nuclei of Plant and Animal Tissues (Plate 



