THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. I 105 



size of the bubbles, however, depends chiefly on the extent of the cut surface 

 at the base of the plant experimented on, although even then the size is not 

 constant in one and the same specimen. 



Although the bubble method is often of service in quantitative experiments 

 owing to its great simplicity, still it is a method confined in practice to aquatic 

 plants. Further, it is impossible with this method to obtain fundamental data 

 of a quantitative kind when absolute, rather than relative values are required. 

 For this purpose eudiometric experiments are necessary, the principle of which 

 is to expose portions of plants, especially foliage leaves, to sunlight in an en- 

 closed chamber filled with an atmosphere rich in carbon-dioxide, and to investi- 

 gate what alterations take place in the composition of the air. Methods of 

 analyses have been perfected in many ways since the time of Bunsen, and 

 have become extremely exact (unnecessarily so for our present purpose) though 

 somewhat complicated ; more recently, however. Bonnier and Mangin have 

 introduced a much improved, and, in its latest form, exceedingly convenient 

 apparatus by means of which gas analysis may be carried out rapidly and 

 without involving laborious reductions (Aubert, 1891). [If Pollacci's criti- 

 cism be correct (Atti Istit. Pavia, 1905), Bonnier and Mangin' s apparatus is 

 perfectly worthless !] The results of eudiometric experiments may be briefly 

 summarized as follows : — The volume of gas remains practically constant while 

 assimilation is going on, because for every volume of carbon-dioxide that 

 disappears an approximately equal amount of oxygen is produced. 



Since the giving off of oxygen is intimately bound up with the assimilation 

 of carbon-dioxide, it follows that there are many other methods of demonstrating 

 the assimilation of carbon over and above the two that have been referred to. 

 Oxygen possesses many properties, some purely chemical, some physiological, 

 which may be employed in determining its presence. The literature on the 

 subject contains many references to its purely chemical properties. Thus 

 Beijerinck (1890) has shown that reduced indigo-carmine becomes blue 

 again owing to the activity of assimilating plants ; Hoppe (1879) placed a 

 plant of Elodea in a sealed glass tube containing a dilute solution of 

 venous blood ; owing to the using up of all the oxygen, the solution then gave 

 the characteristic reaction of haemoglobin ; but the substance at once changed 

 into oxyhaemoglobin (easily recognizable by its spectrum) whenever the tube 

 was brought into sunlight. These methods are useful ones for demonstration 

 purposes, but they have been as little used as methods of investigation as the 

 physiological reaction described by Beijerinck (1901), who showed that if 

 luminous bacteria are brought in contact with green Algae they are phosphor- 

 escent, but only when the green cells are assimilating, and that in the absence of 

 oxygen the luminosity ceases. Another physiological method applicable to re- 

 search hf-s been more often employed, viz. that described by W. Engelmann in 

 numerous memoirs, and summarized finally, under the title of the ' Bacterium 

 method ', in 1894. It depends on the fact that many bacteria, e. g. Bacterium 

 termo, are capable of exhibiting movements in the presence of minute 

 traces of oxygen. If one inoculates a drop of water on a slide with a pure 

 culture of this bacterium, and surrounds the edge of the cover glass with a ring of 

 vaseline to keep out atmospheric oxygen, the bacteria at first exhibit active move- 

 ments in the fluid ; but gradually their power of movement becomes less and 

 less, until, when all the oxygen dissolved in the water has been used up, they 

 at length come to rest. If bubbles of air be enclosed under the cover glass, 

 however, these become centres of attraction to the bacteria in consequence of 

 the presence of oxygen there ; the bacteria move towards the bubbles, collect 

 in their neighbourhood, and continue to exhibit movement for some time, 

 although motionless elsewhere. The phenomena described — and this is a point 

 of importance — are entirely independent of light, for they behave in the same 



