DESM1DIACEJB. 203 



strained through the linen as above described. At first 

 nothing appears on the linen except a mere stain or a little 

 dirt ; but by repeated fillings-up and strainings a consi- 

 derable quantity will be obtained. If not very gelatinous, 

 the water passes freely through the linen, from which the 

 specimen can be scraped with a knife, and transferred to a 

 smaller piece ; but in many species the fluid at length 

 does not admit of being strained off without the employ- 

 ment of such force as would cause the fronds also to pass 

 through, and in this case it should be poured into bottles 

 until they are quite full. But many species of Stauras- 

 trum, Pediastrum, &c., usually form a greenish or dirty 

 cloud upon the stems and leaves of the filiform aquatic 

 plants; and to collect them requires more care than is 

 necessary in the former instances. In this state the 

 slightest touch will break up the whole mass, and disperse 

 it througli the water : for securing them, let the hand be 

 passed very gently into the water and beneath the cloud, 

 the palm upwards and the fingers apart, so that the leaves 

 or stem of the inverted plant may lie between them, and 

 as near the palm as possible ; then close the fingers, and 

 keeping the hand in the same position, but concave, draw 

 it cautiously towards the surface ; when, if the plant has 

 been allowed to slip easily and equably through the fingers, 

 the Desmidiacece , in this way brushed off, will be found 

 lying in the palm. The greatest difficulty is in withdraw- 

 ing the hand from the surface of the water, and probably 

 but little will be retained at first ; practice, however, will 

 soon render the operation easy and successful. The con- 

 tents of the hand should be at once transferred either to a 

 bottle, or, in case much water has been taken up, into the 

 box, which must be close at hand ; and when this is full, 

 it can be emptied on the linen as before. But in this case 

 the linen should be pressed gently, and a portion only of 

 the water expelled, the remainder being poured into the 

 bottle, and the process repeated as often as necessary." 



When carried home, the bottles will apparently contain 

 only foul water ; but if it remain undisturbed for a few 

 hours, the Desmidiacece will sink to the bottom, and most 

 of the water may then be poured off. If a little filtered 

 rain-water be added occasionally, to replace what has been 



