THE SIMPLER ORGANISMS 57 



standing on the window sill. It is not hard to find them in 

 great variety of size and form, and in great beauty and 

 delicacy of organization. 



Closterium (fig. 42) is a very pretty simple alga that is 

 commonly found in the bottom sediment of fresh water 

 ponds. Although very small, its bright green color and 

 crescentic form make it easily recognizable. If we gently 

 lift up from the pond bottom some sticks that have long 

 lain undisturbed, and shake into a white plate filled with 

 water the silt, that covers them, spreading it out in a thin 

 layer, we may usually find Closterium scattered about over 

 the plate. It is visible to the unaided eye, and is easily 

 recognized with a pocket lens. It is easily reared indoors 

 in a cool, well-lighted place in a jar of pond water sup- 

 plied with some mud from the pond bottom, and this is the 

 best way to get a large supply. Enough for class study 

 may usually be obtained by mounting the scrapings of silt 

 from submerged leaves, that have lain long in clear, well- 

 lighted water. 



A few specimens transferred to a slide and examined with 

 a microscope present at once to the eye some important 

 characteristics of green plants. The crescentic plant body 

 is seen to be encased in a transparent capsule, the cell wall, 

 with a green substance filling the greater part of both ends 

 of the crescent, leaving a transparent, clear band across the 

 middle. In this clear band on closer inspection there is 

 seen a slightly granular substance of such transparency it is 

 at first easily overlooked, and in the centre of it is a round 

 body of slightly denser consistency. Although so inconspicu- 

 ous, it is well to fix attention at once upon these latter 

 structures, for they represent the essentials of living struc- 

 ture. The granular mass is protoplasm and the round 

 body within it and forming part of it is the nucleus. The 

 green substance filling and obscuring the protoplasm at the 



