252 



XIX. STRUCTURE OF THE THALLOPHYTA. 



potassium salts and the magnesia, and after suitable dilution add 

 to it the separately-dissolved calcium nitrate. Under these 

 circumstances only a very small portion of insoluble calcium 

 phosphate is separated out. It is not as a rule necessary to 

 add iron, since the traces which come into the culture fluid 

 with the material, and the rain- or tap- water used, will generally 

 suffice. This nutrient solution is used for the culture in such 

 proportion that the culture water contains O2 to O5 per cent, 

 of the salts. If the weights given above are taken as grains, the 

 amounts in question would suffice for about two, or if as grains 

 for thirty, litres of water. 



Many fresh-water algae, e.g., Ulothrix, from rapidly-flowing 

 water cannot be cultivated for long in aquaria, but can be kept 

 in a running spring or fountain, if they are placed in the water 

 while still attached to the stones upon which they were growing. 



The cells of Spiroyyra inajtiscula, when fully developed, are 

 about one and half to twice as long as thick (Fig. 96). The cell- 

 wall is lined by a delicate, 

 colourless, lining layer of 

 protoplasm, which becomes 

 clearly visible if the cells are 

 plasmolysed, i.e., if the pro- 

 toplasmic body of the cell is 

 made to contract by some 

 water - withdrawing medium, 

 such as sugar solution, glycer- 



FIG. 96. Spirof/yra vmjusnita. 1 A cell of ine, solution of common salt, 

 a thread gradually focussed into, showing _ . . 



therefore, besides the chlorophyll bands the or of Saltpetre. To the CO1OUT- 



nucleus with its suspending threads (x 240). lesg Uning layer follow eight 



to ten chlorophyll bands, which usually appear pretty steep and 

 closely wound. The bands have a finely-undulating outline, and 

 are transparent enough to admit of a view into the interior of the 

 cell. At irregular distances in the bands are embedded denser, 

 globular, colourless, bodies the pyrenoids with which we have 

 already become acquainted. These pyrenoids are ensheathed in a 

 hollow sphere of small starch grains. With iodine solution they 



1 The synonymy of the Spirogyra majuscnla of the text is rather obscure. 

 This figure does not resemble S. majiiscula of Kutz. S. orthospira, of Nag. 

 and Archer. The most common of the thick-threaded Spirogyras in Britain 

 is S. nitida. The figure more resembles S. orbicularis of Hassall. [ED. J 



