ROOM-CULTURE OF FRESH-WATER ALG^. 251 



nucleolus, and appears, besides, as if finely granular or finely 

 porous. The alum-haematoxylin or haematin preparations show 

 the nuclei stained dark, and likewise, though more faintly, the 

 pyrenoids. The starch grains are not stained, but on the other 

 hand the microsomata (microsomes) of the cytoplasm are, and 

 almost as darkly as the pyrenoids. 



Spirogyra. Cultivation of fresh-water Alga. The genus Spiro- 

 gyra furnishes us with a simple filament or thread of cells. We 

 choose for examination a species which has a central, readily 

 visible nucleus. So constituted, for example, is Spirogyra majus- 

 cula (S. orthospira), which is met with now and then, not exactly 

 rarely, but sporadically, in pools ; but other species with a central 

 nucleus will serve equally well for examination, and will differ 

 but slightly in the essential relations of their structure. If once 

 in possession of good Spirogyra material you should endeavour 

 to keep it in cultivation. In room cultures Spirogyra is readily 

 spoilt. It is best to keep it in a large vessel, filled with rain or 

 tap- water, in the window of a cool north room. > It must be quite 

 quiet, and left to itself. For the continuous culture of most fresh- 

 water algae it is advisable to use a large aquarium, holding from 

 thirty to forty litres of water. Such an aquarium is made of glass 

 walls in a metal rim. In the middle of the bottom are fixed two 

 upright tubes, of which one is connected with the water supply, 

 the other serves as an overflow. With the former a bent mov- 

 able tube is connected, so that the entering jet of water can be 

 directed to different parts of the aquarium. The mouth of this 

 tube should be situated about 10 cm. under the surface of the 

 water, which latter is determined by the opening of the exit tube ; 

 and this is provided with a suitable cover in order that free 

 swimming algae may not be carried down it by the flowing water. 

 It is generally well if the algae in such an aquarium are fixed in 

 some way, whether by stones, or by individual threads being 

 drawn up on the glass sides so as to dry upon them. In such 

 aquaria the algae are not only kept provided with fresh water, but 

 also subjected to a desirable cool temperature. The direct action 

 of solar rays should always be avoided. 



For special purposes it may be desired to cultivate algae in a 

 nutrient solution. For this, a solution known as " Knop's" may 

 be specially recommended. It consists of calcium nitrate 4 parts, 

 magnesium sulphate 1 part, potassium nitrate 1 part, potassium 

 phosphate 1 part. In preparing it, we first dissolve the two 



