io6 METABOLISM 



way whether the shde be placed in the dark or in the hght. If in such a pre- 

 paration we replace the air-bubbles by specimens of a unicellular alga we find that 

 if kept in the dark all the bacteria quickly come to rest ; as soon as the preparation 

 is illuminated, however, the cells of the alga give off oxygen, and the oxygen 

 at once exerts an influence on all the bacteria that happen to be near the cells, caus- 

 ing them to rush towards them and to move actively in their immediate vicinity 

 (Fig. 23,/,//). When the light is removed the bacteria again distribute themselves 

 over the field and come to rest, while each successive illumination once more 

 induces movement and crowding round the alga. This method owes its 

 frequent use more especially to its very great sensitiveness, for the presence of 

 the minutest traces of oxygen may be demonstrated by its means. One may 

 vary the sensitiveness by emplojdng other bacteria, e. g. Spirillum, as well as 

 organisms or cells belonging to other groups (Infusoria, Flagellata, or sperma- 

 tozooids of sea urchins) since many of these react, some only to larger, some to 

 even smaller quantities of oxygen than Bacterium termo. On the other hand, the 

 method has its drawbacks ; it must in any case be used with caution, as we 

 shall have occasion to see by and by. 



By means, then, of the methods which have been described it is possible 

 to prove that green parts of plants can assimilate carbon-dioxide in light, and 

 this is one of the best established facts in plant physiology. Nor is it difficult 

 to show that this power is confined to green organs. Every experiment with 

 a fungus or with a root demonstrates at once absence of any decomposition of 

 carbon-dioxide ; while, on the other hand, the parts of the plant with the 

 darkest green colour, the foliage leaves, have long been known to be the most 

 active members in carbon-dioxide assimilation. It is true that carbon-dioxide 

 assimilation has also been often observed in parts of the plant otherwise coloured, 

 but more careful study has always shown that these parts contain green colouring 

 matter, which is simply masked by other pigments. The carriers of the green 

 colour are the chloroplasts, which are special organs of the cell, capable of 

 increasing in number by division, and developing the green colour under certain 

 conditions. Thus it is possible to prevent its formation by omitting iron from 

 the nutritive supply (compare p. 85), and also, in the higher plants at least, 

 by keeping the plant in the dark. In both these cases the chloroplast itself — 

 the protoplasmic basis of the chromatophore — is formed, but the chlorophyll 

 does not develop ; the chloroplast remains either colourless or yellow. Further, 

 Pfeffer (1881) has shown by the eudiometric method, and Zimmermann 

 (1893) has confirmed his results by the bacterium method, that parts of plants 

 which have become chlorotic owing to the absence of iron are quite unable to 

 decompose carbon-dioxide, and hence we must look upon the green pigment 

 as a factor of the highest importance in assimilation, and all the more so because 

 we find that etiolated plants grown in the dark cause no decomposition of 

 carbon-dioxide when first placed in the light. Decomposition will, of course, take 

 place after a certain length of time, since the green colour becomes rapidly 

 developed in light. Recently, Ewart (1897) has made certain observations 

 on etiolated cells, and has shown that if they be not too old or too young, an 

 evolution of oxygen from them may be demonstrated by the bacterium method, 

 even before the slightest trace of green colour has been developed. This would 

 appear to confirm an older research of Engelmann's, but further investigations 

 are required to determine whether the interpretation Ewart puts on his observa- 

 tions is correct or not. Further, it must be noted that an attraction of 

 bacteria does not take place on every occasion. Bacteria respond by mobility, 

 for instance, to other substances besides oxygen (Lecture XLIII), and it is im- 

 possible to affirm that etiolated chloroplasts may not contain substances which 

 have an attractive influence on such organisms. At present we may be per- 

 mitted to ignore Ewart' s statements, and say that only cells containing 



