100 ACTION OF LIGHT AND HEAT 



Mrs. B. I doubt whether the term sleep be literally 

 appropriate to that state of relaxation and inaction which 

 appears to afford them repose during that season. The 

 leaves and flowers usually change their position as soon 

 as it grows dark ; in many plants the leaves drop ; in 

 others they close, as well as the petals of the flowers, and 

 are opened by the first rays of the morning sun. The 

 leaves then recommence their chemical operations, the 

 spongioles draw up a provision for their labors, every 

 function which had ceased or diminished during the night 

 is again renewed and the whole plant reanimated. It is 

 this effect, produced by light on plants, which I call be- 

 ing awakened. 



Secondly, the direct rays of the sun are necessary to 

 enable plants to decompose carbonic acid gas in any sen- 

 sible quantities. We have already observed, that in this 

 process the oxygen of the carbonic acid is exhaled by the 

 leaves, and the carbon deposited in the plant : now, it is 

 this deposition which produces their green color. Mr. 

 Sennebier is of opinion, that carbon is not positively 

 black, but of a dark-blue color. The cellular tissue of 

 plants is of a yellowish white ; consequently, when those 

 minute blue particles are lodged within the yellow cells, 

 the combination of the two colors produces green, in 

 which the blue or yellow tint prevails, in proportion as 

 the carbon or cellular tissue predominates. 



Caroline. That is very curious, and accounts for the 

 pale delicate tint of the spring verdure, when but a small 

 quantity of carbon has been deposited in the leaves ; and 

 the deeper shades which plants acquire in summer and 

 autumn, when they have accumulated a greater stock of 

 carbon. 



But what is it that produces the change of color at the 

 fall of the leaf, and, indeed, often takes place previous ta' 

 their fall, when some leaves assume a beautiful red or yel- 

 low color ? 



Mrs. B. Some ingenious experiments have lately been 

 made on this subject by Mr. Macaire. He ascertained 

 that, late in the autumn, when leaves begin to change 



554. When it grows dark, what change takes place in flowers and 

 leaves'? 555. What does Mrs. B. mean by the awakening of plants'? 

 556. What is the second way in which light operates on plants'? 557. 

 How is the green color of plants produced'? 558. In what way did 

 Mr. Sennebier account for if? 559. For what does Caroline suppose 

 his hypothesis will account 1 



