XI.] SPIROGYRA. 405 



species its form is that of a biconvex lens, as 

 described in the text. Inside it is the nucleolus, 

 The nucleus is surrounded by a layer of pro- 

 toplasm. Observe that strands of protoplasm 

 radiate from the latter, crossing the cavity of 

 the cell, and attached at their outer ends to 

 those parts of the chromatophores where the 

 pyrenoids occur. 



. The minute structure will be better made out by 

 means of staining. Place some Spirogyra in satu- 

 rated solution of Picric acid 1 for not less than 

 12 hours. Then wash thoroughly in distilled 

 water. The chlorophyll will now have entirely 

 disappeared. Stain the filaments thus prepared 

 with Borax carmine solution 2 for an hour or more. 

 Then mount in dilute glycerine 3 and examine with 

 the highest power available. The cell-wall will 

 be unstained. The protoplasm of the primordial 

 utricle, chromatophores, and radial strands will 

 have taken the stain very lightly. On the other 

 hand the nucleus will be decidedly, and the nu- 

 cleolus very deeply, coloured. The pyrenoids 

 stain deeply, and their crystalline form can now 

 be clearly made out. The layer of starch-grains 

 surrounding each pyrenoid remains quite colour- 

 less. It is especially in material prepared in this 

 way that the relation of the radial protoplasmic 

 strands to the pyrenoids can be traced. 



Tj. The process of cell-division, as described in the 

 text, can be best observed by keeping the Spiro- 

 gyra at a low temperature during the night, and 



1 Appendix, E. 



2 Appendix, E. 



3 The best plan is to mount in water and gradually run in glycerine 

 under the cover-slip. 



