LIGHT AND VEGETATION 81 



energy of illumination, measured with the thermocouple, 

 although its composition was changed. 



These details gives some indication of the care that must 

 be taken in the comparative study of the influence of different 

 colours. Note in this connection that to equalize the energies 

 corresponding to the different colours the illumination must 

 be reduced. Experiments of this kind always have one fault 

 in common ; they are made only in much worse conditions of 

 illumination than the normal conditions of existence of the 

 plant. The coloured illumination in Sierp's experiments is 

 ridiculously low when compared with the brilhance of full 

 sunlight. Nevertheless, these experiments, following many 

 which had not been so thoroughly carried out by other 

 investigators, were the first to give conclusive and certain 

 results. 



In short, the stomata open under visible hght. Since the 

 spectral compositions are very complex, it is impossible to 

 state precisely the limits of the range of wave-lengths in which 

 this action is produced. In addition, the blue causes a more 

 rapid and wider opening than the red — the open surfaces are 

 in the ratio of 5 to 3. 



Other investigators, before Sierp, had stated the contrary 

 — that the red was more effective than the blue — but this was 

 because they had omitted to measure the energy of the 

 illumination in the two cases. Their observations were made 

 in very low blue illumination and very intense red; the con- 

 ditions were not identical and the intensity of the red largely 

 compensated for its lower efficacy. In reality, with equal 

 energy, blue hght stimulates the opening of the stomata to 

 a much greater extent. 



One effect must be that the gaseous exchanges between 

 the leaf and the atmosphere, and particularly the evaporation 

 of water, are faciUtated. If we also remember that the blue 

 and the violet are absorbed in greater proportions in the 

 tissues of the leaf than any other visible radiations, we see 

 that there are two convergent causes tending to favour trans- 

 piration under the influence of blue hght. This is precisely 



