HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 93 



Hoffman's blue to a few drops of strong sulphuric acid in 

 a watch-glass, stir with a glass rod ; place sections in the 

 liquid for a few minutes, rinse them in water, and mount 

 in glycerine ; the continuity of the sieve-tube contents is 

 made clear by this treatment. 



(e) The companion-cells are made conspicuous by the 

 deep staining of their contents by aniline blue. This also 

 gives a good double stain in conjunction with safranin; 

 place sections in safranin for 15 or 20 minutes, rinse in 

 alcohol and transfer to aniline blue for about a minute, 

 rinse again, dehydrate with absolute alcohol, clear with 

 clove oil, mount in balsam. The lignified tissues (xylem 

 and sclerenchyma) are stained red, the remaining tissues 

 (with cellulose walls) blue. 



102. Development of Vessels, etc. In transverse and longi- 

 tudinal sections through the youngest parts of the Marrow stem, 

 notice that the ground tissue is complete right across the stem ; the 

 cavity found in the older parts is formed by the central region of 

 ground tissue (pith) becoming torn as the stem grows thicker. Also 

 notice that the wood contains only the narrow spiral and ringed 

 vessels, and that the collenchyma and sclerenchyma are not yet dis- 

 tinguished sharply from the ordinary ground tissue. 



103. Maceration of Tissues. Cut out about 1 cm. of 

 Marrow stem, chop it by radial cuts into pieces including 

 each a bundle, and heat the pieces for a few minutes in 

 Schultze macerating fluid ( 121). Wash in water, mount 

 in glycerine, tease with needles, or press on the cover-glass, 

 and note the isolated tissue constituents cells, fibres, 

 vessels of various kinds. Compare carefully with the 

 appearance of the tissues as seen in transverse and longi- 

 tudinal sections of the stem. 



In herbaceous structures like Marrow stem, the tissue 

 constituents can be isolated by using (instead of Schultze 

 fluid) a mixture of one part hydrochloric acid and three 

 parts alcohol ; place sections in this mixture for a day, 

 rinse in water, treat with potash, when the cells, etc., are 

 readily dissociated by pressure under the cover-glass. 



Herbaceous stems are readily macerated by chromic acid. 

 Place sections in strong solution of this acid for a few 



