394 PRACTICAL BOTANY 



F. Note the dark olive-brown colour of the whole 

 thallus when fresh. Place a piece of the fresh thallus 

 in methylated spirit : in a short time it will assume the 

 bright green colour characteristic of chlorophyll : this 

 is due to the fact that the brown colouring-matter 

 (phycophsein, or phycoxanthin) characteristic of the 

 PhaaophyceaB, masks the chlorophyll in the natural 

 state ; but, being more readily soluble than chlorophyll, 

 it is first removed, and the chlorophyll is then 

 apparent. 



MICROSCOPIC EXAMINATION 



II. As it is almost impossible to make satisfactory 

 preparations of the tissues of Fucus from fresh material, 

 it will be found a great advantage to fix and harden 

 them, by some such method as that described in small 

 type on page 6 : by this method the material is fixed 

 by treatment with a solution of picric acid in sea- 

 water, and after washing, to remove excess of picric 

 acid, it is hardened in successive strengths of alcohol. 

 The length of time of immersion in the picric acid 

 should be proportional to the bulk of the tissue, 

 so that the reagent may penetrate throughout the 

 tissue. 



An alternative method, which gives good results, is to treat the 

 specimens first with 1 per cent, chromic acid, wash with water, 

 and harden in successive strengths of methylated spirit. 



From material thus prepared select a young flattened 

 branch of the thallus in which the midrib is but slightly 

 marked : cut transverse sections from it, mount in pure 

 glycerine, and observe under a low power 



