INTRODUCTION. 39 



cautiously towards the surface, when, if the plant has been 

 allowed to slip easily and with an equable movement through 

 the fingers, the Desmidiese, in this way brushed off, will be 

 found lying in the palm. The greatest difficulty is in with- 

 drawing 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 contents of 

 the hand should be transferred at once 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 re- 

 peated as often as necessary. 



Sporangia are collected more frequently by the last than 

 the preceding methods. When carried home, the bottles 

 will apparently contain only foul water, but if it remain un- 

 disturbed for a few hours, the Desmidiese will sink to the 

 bottom, and most of the water may then be poured off. If 

 a little fresh water be added occasionally to replace what has 

 been drawn off, and the bottle be exposed to the light of the 

 sun, the Desmidiese will remain unaltered for a longtime. I 

 have now before me some specimens of Euastrum insigne, the 

 fronds of which are in as good condition as when I gathered 

 them at Dolgelly five months ago. 



Mr. Thwaites's kindness has enabled me to render this 

 Introduction more complete by the following account of two 

 methods adopted by him in mounting minute Algse for the 

 microscope, which he has drawn up at my request. The 

 remarks which I have appended have been derived from 

 other sources, as well as from my own experience : — 



" In making preparations of the Algse for the microscope, 

 there are two things which principally require to be attended 

 to : first, to obtain a fluid which shall preserve the plant 

 as little altered as possible from its appearance when living ; 

 and secondly, to adopt the best means for preventing the 

 escape of this fluid after the object has been mounted in it. 



