DESMIDACE^. 415 



obtaining specimens is very great, it will materially assist the efforts 

 of the microscopist to learn the method adopted by Mr. Ralfs, Mr. 

 Jenner, and Mr. Thwaites. " In the water the filamentous species re- 

 semble the Zygnemata ; but their green colour is generally paler and 

 more opaque. When they are much diffused in the water, take a piece 

 of linen, about the size of a pocket handkerchief, lay it on the ground 

 in the form of a bag, and then, by the aid of a tin box, scoop up the 

 water and strain it through the bag, repeating the process as often as 

 may be required. The larger species of Euastrum, Micrasterias, Clos- 

 terium, &c., are generally situated at the bottom of the pool, either 

 spread out as a thin gelatinous stratum, or collected into finger-like 

 tufts. If the finger be gently passed beneath them, they will rise to 

 the surface in little masses, and with care may be removed and 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 considerable 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 employment 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 Staurastrum, 

 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 through the water. I would recommend the following 

 method as the best adapted 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 in- 

 verted 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 Des- 

 midacea3, in this way brushed off, will be found lying in the palm. The 

 greatest difficulty is in withdrawing the hand from the surface of the 

 water, and probably but little will be retained at first ; practice, how- 

 ever, will soon render the operation easy and successful. The contents 

 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. 



