80 LIGHT, VEGETATION AND CHLOROPHYLL 



reveal a hitherto unsuspected secret of the mysteries of plant 

 life. The more science advances, the better we shall be able 

 in agriculture to use our knowledge to increase production 

 and improve quaUty. 



Opening of the Stomata and Transpiration 



Transpiration is stimulated by visible radiation, as it is 

 by infra-red, but while in infra-red alone the stomata of the 

 leaves remain closed and evaporation must thus take place 

 through the epidermis, visible radiations, blue in particular, 

 have the effect of opening the stomata. As a result, it appears 

 that transpiration is still further stimulated. 



A very thorough study of the opening of the stomata 

 was made by Sierp in 1933. He had no difficulty in con- 

 firming the already known fact that under infra-red light the 

 stomata remained closed. Working on the leaves of Helianthus 

 annuus, grown in a pot, and without detaching them from the 

 plant, he examined their lower surface under the microscope. 

 A few stomata were chosen and their area was measured. 

 The same stomata were observed throughout the experi- 

 ments. This was necessary because different stomata behave 

 differently, even on the same leaf. 



The variation of the area, with time, was measured while 

 the plant was being submitted to a given illumination, 

 supplied by the filtered radiation from an incandescent lamp. 

 A vessel containing a 6 per cent solution of copper sulphate, 

 1 cm. thick, served as a screen opaque to the infra-red ; then 

 coloured glasses allowing only spectral bands to pass were 

 used ; these bands were rather wide, but they presented a maxi- 

 mum of energy in the blue, the green, the yellow, the orange or 

 the red. With the blue filter, the power transmitted was rather 

 small, as the incandescent lamp produces little radiation of 

 that colour; the power of the illumination varied, in different 

 cases, from 0-1 cal./sq. cm. /minute to half that figure. Sierp 

 reduced the luminous flux transmitted by the other filters to 

 bring the illumination obtained with them to the same level, 

 so that the plant should always be submitted to the same 



