240 THE INFLUENCE OF LIGHT ON LIFE. 



colouring matter are not developed. From this we can see that 

 light has a good deal to do with the distribution of plants, as, 

 unless they are provided with protective appliances, they will never 

 be able to grow in any situation where the light exceeds a certain 

 fixed standard. 



The weed which grows in ponds is often of a less green colour 

 on the surface of the pond than it is a little beneath the surface, 

 and this is due to the bleaching action of the sun, for when the 

 light is very strong the chlorophyll loses its bright green colour. 

 If a piece of black paper be fixed on a green leaf exposed to 

 strong light, it will protect the part so covered, and when the paper 

 is again removed the places which were uncovered will be seen to 

 be quite pale in comparison to the part which was sheltered. It is 

 to this bleaching action of light that the disappearance of the 

 green in autumn is said to be due. I steeped several ivy leaves in 

 methylated spirits and obtained a beautiful bright green solution, 

 due, of course, to the chlorophyll having been dissolved out of the 

 cells. Part of this solution was then exposed to the light, and in 

 a day or two I found that the green colour had quite disappeared, 

 and that the liquid had assumed a brownish hue. This, then, is 

 the bleaching action just referred to, and which is very noticeable 

 in cabbage and other leaves which have become separated from 

 the stock. 



Some plants turn the edges of their leaves to the light when it 

 is too strong, and when the quantity is agreeable they turn the flat 

 surfaces to the lights an arrangement which, like other contrivances 

 to avoid undue light — such as the folding up of leaves, etc. — keeps 

 down at the same time an unnecessary waste of moisture, for 

 it is through the light that the stomata are caused to open 

 and allow the process of exhalation to proceed. As too little 

 light tends to impede exhalation, too much light tends to cause 

 danger in exactly the opposite direction. It is light which causes 

 unripe or acid fruit to be changed into ripe or sweet fruit ; and it 

 is light, as we so well know, that controls the opening and closing 

 of the daisy, or "day's eye," and so many other flowers of the 

 field. 



Experiments have been made with a view of determining which 

 rays of the spectrum produce the greatest influence on plants, and 



