LECTURE VI. 



YEAST lives surrounded with food materials so that the source of 

 the substances which it incorporates as its food is not far to seek. 

 The supply of food for an organism like Chlamydomonas is not 

 so obvious. The rain-water in which it lives contains no proteins 

 and no carbohydrates. This water is practically pure, though it 

 may contain traces of certain mineral salts derived from the slight 

 solubility of the mineral particles of the soil forming the sides of 

 the pool or of the mineral dust falling into it. In this way the 

 water surrounding Chlamydomonas may contain traces of sul- 

 phates, bicarbonates, chlorides, nitrates, phosphates, existing as 

 salts of magnesium, calcium, sodium, potassium, and iron. Car- 

 bon dioxide is also dissolved in this water. 



If a sample of Chlamydomonas be observed when exposed to 

 strong light, it will be noticed that a froth of minute bubbles is 

 formed at the surface. The gas contained in these bubbles is 

 oxygen. Furthermore if the cells are microscopically examined 

 before and after this frothing has occurred it will be found that 

 the amount of starch in the pyrenoid has noticeably increased. 

 To form this starch (C 6 H 10 O 5 ) M , which is a carbohydrate, evidently 

 a supply of carbon must be available. Experiment shows that 

 the frothing and the formation of starch only take place when the 

 Chlamydomonas is exposed to light and when a supply of carbon 

 dioxide dissolved in the water is available. 



The conditions for the formation of starch may be more easily 

 studied in one of the higher water plants. I have here put a 

 handful of a very common fresh-water weed Elodea canadensis 

 into this large glass vessel containing water in which carbon 

 dioxide is dissolved. Bubbles will be seen to rise from the 

 surface of the plant as soon as it is exposed to a bright light. 

 This inverted funnel is now arranged over the weed to catch the 

 bubbles and they, guided in their upward course, are caught in 

 this inverted test tube previously filled with water. The bubbles 

 displace the water in the tube as they accumulate. Thus collected 

 the gas may be tested. It will be found to be nearly pure oxygen. 



