The Tropics and Pigment 365 



The synthetic properties of chlorophyll depend chiefly upon 

 its chemical constituents, and no doubt the physical properties of 

 the pigment may enter largely into effecting the changes that 

 take place in the chloroplasts. But we are not concerned with 

 either the chemical or vital processes themselves, we are only 

 dealing with the physical properties of chlorophyll as a pigment, 

 and are more particularly concerned in showing that it has the 

 power of controlling the activities of light and heat rays ; and 

 this will be readily conceded if we attempt to explain the ordin- 

 ary phenomena that occur in the course of plant life. 



Young plant life is vigorous, and radiant energies are freely 

 demanded for the purposes of rapid growth. Here it will be 

 observed that the deposition of pigment is least. As the season 

 and growth advance the deposit increases, which in turn must 

 lessen the chemical activity of light, and at the same time reduce 

 the diathermancy of the tissues. This means the conversion of 

 more of these radiant energies into actual heat, till at the end 

 of the season or of growth the chlorophyll has become so dense 

 that it will, for the greater part, if not wholly, transform these 

 energies into heat. With annual plants, as in grain crops, the 

 pigment changes its colour to a yellow pigment, xanthophyll. 

 This pigment has the power of further reHucing actinic energy 

 to heat, and at the same time, in some measure restoring the 

 transparency of the tissue and allowing the transmission of 

 radiant heat, which in due course completes the ripening of 

 the grain. With the deposition of pigment, the growth of 

 perennial foliage is completely arrested, as with the ivy, but 

 by this*means these leaves are able to secure the necessary heat 

 that sustains them through the winter months, while in the 

 summer desiccation is avoided by the transpiration of fluids. 



Here again experimental evidence can be given in support by 

 the translumination of the various coloured vegetable mem- 

 branes. 



It is not necessary to give evidence that Nature protects plant 

 life according to the needs determined by its environment, nor 

 is it required to prove the fact that British trees, like British 

 people, find the struggle of life increases adversely as they near 

 the equator or the poles. 



