38 INTRODUCTION, 



111 the water the filamentous species resemble the Zyg- 

 nemata, but their green colour is generally paler and more 

 opake. They often occur in considerable quantity, and, 

 notwithstanding their fragility, can generally be removed by 

 the hand in the usual manner. When they are much dif- 

 fused in the water, I 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, Mi- 

 crasterias, Closterium, &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 ap- 

 pears on the linen except a mere stain or a little dirt ; but by 

 repeated fiUings-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 di- 

 sperse it through the water. I would recommend the fol- 

 lowing 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 invested 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 



