5 86 MICROSCOPIC FORMS ( )F VEGETABLE LIFE THALLOPHYTES 



constriction between the two halves of the cell, the division of its 

 margin into segments by incisions more or less deep, and its exten- 

 sion into teeth or spines. 



The Desmldiacece are not found in running streams, unless the 

 motion of the water be very slow, but are to be looked for chiefly 

 in standing waters. Small shallow pools that do not dry up in 

 summer, especially in open, exposed situations, such as boggy 

 moors, are most productive. The larger and heavier species 

 commonly lie at the bottom of the pools, either spread out as a 



thin gelatinous stratum, 

 or collected into finger 

 like tufts. By gently 

 passing the fingers be- 

 iieath these they may be 

 caused to rise towards the 

 surface of the water, and 

 may then be lifted out by 

 a tin box or scoop. Other 

 species form a slimy 

 stratum floating on the 

 surface of bog- pools, or a 

 greenish or dirty cloud 

 upon the stems and leaves 

 of other aquatic plants ; 

 and these also are best 

 detached by passing the 

 hand beneath them, and 

 ' stripping ' the plant be- 

 tween the fingers, so as to 

 carry off upon them what 

 adhered to it. If, on the 

 other hand, the bodies of 

 which we are in search 

 should be much diffused 

 through the water, there 

 is no other course than to 

 take it up in large quanti- 

 FIG. 440. Binary subdivision and conjugation of ties by the box Or SCOOp 

 Desmidium cylindricum : A, portion of filament, and to separate them by 

 surrounded by gelatinous envelope : B, dividing e t ,:: fVii-nucrli a 'an 

 cell; C, single cell viewed transversely; D, two stia ; inm g tlllOUgil a pie 

 cells in conjugation ; E, formation of zygospore. OI linen. At first, nothing 



appears on the linen but 



a mere stain or a little dirt ; but by trie straining of repeated 

 quantities a considerable accumulation may be gradually made. 

 This should then be scraped off with a knife, and transferred 

 into bottles with fresh water. If what has been brought up 

 by hand be richly charged with these forms, it should be at 

 once deposited in a bottle ; this at first seems only to contain 

 foul water ; but by allowing it to remain undisturbed for a 

 little time, the desmids will sink to the bottom, and most of 

 the water may then be poured off, to be replaced by a fresh 



