Ohservaiio)is on Spirogyra. 425 



After the Spirogyra has began to grow in the vessel, which 

 it will have done after 24 hours, we supply it daily early in 

 the morning for two minutes with a stream of carbon 

 dioxide bubbles, avoiding any undue commotion in the water, 

 and thereby mixing of the healthy threads with the decay- 

 ing ones lying at the bottom of the vessel. Every week 

 the vigorous Spirogyra should be lifted out carefully and 

 placed in another vessel, to allow removal of the debris lying 

 at the bottom of the vessel. 



WTiat effect an extra supply of carbon dioxide has on 

 carbon assimilation the following experiment shows clearly: — 

 Take two glass jars, 4 inches broad by 2 feet high, fill them 

 with ordinary water up to 6 inches from the top, pass through 

 the water in vessel A a stream of carbon dioxide for five 

 minutes, while the water in vessel B does not receive any 

 COo in addition to that already present in the water. Next 

 take an equal quantity of vigorously-growing Spirogyra and 

 push it to the bottom of vessels A and B, and expose these 

 to bright daylight. After a varying time, according to the 

 strength of the light, the material in vessel A will rise to the 

 surface of the water a considerable time before the material in 

 vessel B will do so, owing to the fact that assimilation, going 

 on faster in vessel A (containing an extra amount of COo), 

 a greater number of oxygen bubbles will be set free, which, 

 acting as buoys, will carry the Spirogyra threads upwards. 



By a process similar to the one just described the patches 

 of Spirogyra that grow on the surface of ponds, &c., are 

 brought about ; for whenever the threads become entangled, 

 and bubbles of oxygen that are given out during assimilation 

 cannot escape through the entangled mass, the gas will 

 gather till it is strong enough to overcome the resistance of 

 the threads mooring the mass to the bottom of the pond, the 

 foot-end of the threads will break, and, as already stated, a 

 floating mass of Spirogyra will result. 



The apices of threads grown in a glass jar have a nutative 

 power, as will become evident from fig. 2, Plate II., which 

 represents a mass of Spirogyra in the shape of a band, the 

 broad side being directed towards the source of light. The 

 band does not grow upwards, however, in a straight line, for 

 at a and hj the threads have grown towards the source of 

 light, or rather, after cell division had taken place during the 



