Watson on Structure and Relation of the Plastid. 337 



or their supposed homologues in a series of plants, beginning 

 with those low in the scale of evolution, and ascending to 

 higher and more specialized types might prove suggestive. 



Collection and Fixation Methods. — As a simple and useful 

 material, I was fortunate in discovering on some Nitella 

 an abundant growth of a parasitic Colcochactc (C solnta). 

 Alcohol of 30 per cent, strength was used as a killing fluid, 

 and the percentage was gradually increased up to 70 per 

 cent. Similar treatment was adopted for Oscillaria, Clado- 

 phora and Zygncma. Dififerent fluids were used for Spiro- 

 gyra. After repeated experiments, mercuric chloride 

 proved most satisfactory. In the use of this agent, subse- 

 quent staining was unnecessary, as sufficient dififerentiation 

 was secured by the action of the mercuric salt. For Nitella, 

 weak alcohol proved most satisfactory. Gardiner's sulphuric- 

 acid-iodine method for the demonstration of protoplasmic 

 continuity proved of value, but a weak solution only of the 

 acid was used for a short time. 



Anthoccros was killed in several fluids, the most satis- 

 factory being Flemming's compound and i per cent, chromic 

 acid. Of the two chromic acid is preferable, as giving the 

 clearest and least altered details. Among the mosses 

 Funaria was chosen, and the best results were secured with 

 I per cent, chromic acid. A variety of fluids was used on 

 fern prothallia, and sulphuric-acid-iodine treatment gave 

 very pleasing results. The delicate prothallia, after being 

 killed in Flemming's fluid and subsequently washed, were 

 dipped quickly in dilute sulphuric acid solution, then washed 

 and placed in iodine solution, rewashed, stained and mounted 

 in I per cent, acetic acid. 



Of the Lycopodineae, Psilotum was selected on account 

 of its large nuclei and plastids. Various methods were used, 

 Flemming's fluid serving best, as a fixative. Serial micro- 

 tome sections were also prepared in the usual manner. 

 Amongst Monocotyledons, Cypripcdiwn insigne and Dieffen- 

 bachia were chosen, owing to the large size of the plastids. 



