FKESHWATER WEEDS. 185 



and transparent, apparently without a limy coating. 

 There are several other species, but I have not found 

 them. The most curious circumstance in the history of 

 Chara is their circulation. The grains of coloured matter 

 in each joint are disposed in two spiral bands, so as to 

 leave an uncoloured space between. A circulation of 

 the coloured mass takes place from below to the top of 

 the cell, and then down again. If the joint is tied in the 

 middle, then the fluid of each half circulates in the same 

 way. This phenomenon may be observed in the shining 

 Chara by the use of the microscope. It is said that the 

 Malaria, hoveriug; over the Pontine and other marshes, 

 is caused by the odour of sulphuretted hydrogen emitted 

 by Characese. 



The fresh-water tribes of green Algae are most numer- 

 ous and pervasive. Professor Harvey thus writes of 

 their habitats and uses : — " The Chlorosperms are more 

 widely diffused than any other Algae. A comparatively 

 small number are found in the waters of the sea, A far 

 larger portion inhabit fresh- water rivers, lakes and 

 ponds, ditches, bog holes, the gutters of houses, and 

 sewers : in fact, anv where that fresh or unfresh water 

 may lie ; nor are they absent from the hot springs of 

 volcanic regions ; and are capable of vegetating wher- 

 ever moisture and a moderate temperature prevail. 

 Thus universally dispersed, they answer many a good 

 purpose in the household of nature ; and are specially 

 useful in purifying the water in which they live. Un- 

 sightly as the green scum may be which they form on 

 its surface, the growth is a renovating process, in which 



