128 



HEALTH AND DISEASE 



and the chlorophyll-green alga (fig. 481). The first gives off a small 

 amount of oxygen insufficient to exert any useful oxidizing function, while 

 both excrete fetid gases during their decomposition; consequently, when 

 found in water, they may be taken as an indication that it is unfit for 

 use. Of the second class, one family (Conjugate) is distinguished by the 

 peculiarity of extreme sensitiveness to the influence of decomposing sub- 

 stances. They can only live in water which is charged with oxygen. 

 Their presence, therefore, in a flourishing condition may be accepted as 



6 



Fig. 481.— Chlorophyll-green Alga 

 ( Vaucheria sessilis) 



1, Plant. 2, Portion of filament en- 

 larged. 3, Swarm spore. 4, Portion of 

 the same enlarged. 



Fig. 482— Veil vox globator (colony) 



1, Antheroid. 2, Oosperm (mature). 3, Oogone. 

 4, Peripheral cells. 5, Antherozoids. 6, Mode of 

 division of parent cell of a zoosporange. 



proof that the water is free from any large amount of objectionable organic 

 constituent. It may be further noted of these plants that while they 

 demand a large quantity of oxygen as a condition of their own life, they 

 give off a considerable quantity of the same gas, to the manifest benefit 

 of the water in which they reside. 



An illustration of the purifying influence of plant-life on water is quoted 

 as having occurred in India some years ago, when, by some accident, all the 

 aquatic plants were removed from the water-tanks. The consequence was 

 that the previously wholesome water quickly became unfit for consumption. 

 According to Rafter, writing in the transactions of the American Society of 

 Civil Engineers, the well-known and extremely beautiful alga, the Volvox 



