168 PLANT PHYSIOLOGY 



parts of the plant can absorb carbon dioxide and liberate oxygen. 

 A direct proof was obtained by Engelmann by means of a very 

 sensitive bacterial method for detecting oxygen. Many bacteria 

 show mobility only in the presence of oxygen, even in small 

 quantities, and will move toward the source of oxygen. If a 

 unicellular alga is placed in a drop of liquid containing such 

 bacteria and the preparation is exposed to light, all the bacteria 

 will gather around the alga as a source of oxygen. Care must be 

 taken, however, to coat the edges of the cover glass with vaseline 

 to prevent the diffusion of oxygen from the surrounding air. 

 If instead of a single-celled alga, a thread of Spirogyra is studied, 

 the chloroplast of which has the form of a spiral band with large 

 portions of hyaline protoplasm between the coils of this spiral, 

 an accumulation of bacteria will be observed near such parts 

 only of the cell wall as are contiguous to the spiral chloroplast. 

 The movements of bacteria will be observed near the chloroplasts 

 that have been separated from destroyed cells, though, in this 

 case, the activity of the chloroplasts continues but a short time, 

 after which they die. Chloroplasts consist of a protein body, 

 called the ''stroma," impregnated with several pigments, among 

 which may be mentioned two green pigments of chlorophyll, 

 chlorophyll a and chlorophyll 6, and two yellow pigments, 

 carotene and xanthophyll. 



For assimilation, the harmonious and concordant functioning 

 of both stroma and chlorophyll is absolutely indispensable. This 

 is obvious from the fact that it has never been possible to repro- 

 duce the decomposition of carbon dioxide in solutions of chloro- 

 l)hyll in various kinds of liquids, alcohol, benzine, oil, etc. On 

 the other hand, the colorless plastids (leucoplasts) as well as the 

 yellow pigments are not capable of photosynthesis. 



The importance of chlorophyll in the process of carbon assimi- 

 lation will be discussed further on. It Ynay be stated here that 

 it renders the energy of the absorbed light effective in the decom- 

 position of carbon dioxide. In all probability, it takes part in 

 this process chemically also, entering into combination with 

 carbon dioxide and making it accessible to the action of light. 

 The primary importance of the stroma consists in forming the 

 structure through which the particles of chlorophyll are dis- 

 tributed and which, in a way not yet understood, makes their 

 functioning possible. It is also the carrier of enzymes that 



