Ch. X. | LEAVES. APPENDAGES. 59 



to the side on which the greatest quantity of light is to be 

 found. It has already been observed, that plants throw off oxy- 

 gen gas ; but for this purpose they require the agency of light. 



233. Carbonic acid gas is a necessary food of plants ; this 

 consists of carbon and oxygen, and is decomposed by the agency 

 of light ; the carbon becomes incorporated with the vegetable, 

 forming the basis of its substance, while the oxygen is exhaled 

 or thrown off into the atmosphere. 



234. Many plants close their leaves at a certain period of the 

 day, and open them at another ; almost every garden contains 

 borne plants, in which this phenomenon may be observed ; it is 

 particularly remarkable in the sensitive plant, and the tamarind 

 tree. The folding up of leaves at particular periods, has been 

 termed the sleep of plants; this may seem a singular term to 

 apply to plants ; but a celebrated botanist remarks, " this folding 

 up of the leaves may be as useful to the vegetable constitution, 

 as real sleep is to the animal." 



235. Linnaeus was led to observe the appearance of plants in 

 the niirht, from the following circumstance, which occurred in 

 raising the Lotus plant ; he found one morning some very thrifty 

 flowers, but at night they had disappeared ; this excited his at- 

 tention, and he began to watch the plants through the night, in 

 order to observe the period of their unfolding. He was thus 

 led to investigate the appearance of other plants in the night 

 and to observe their different manner of sleep. He found that 

 some folded their leaves together, some threw them back upon 

 their stems, or exhibited other curious appearances. This phe- 

 nomenon has been attributed to the absence of light. 



236. The following experiment was once made by a botanist; 

 he placed the sensitive plant in a dark cave at midnight, and 

 then lighted up the cave with lamps; the leaves which were be- 

 fore folded up suddenly expanded, and when on the following 

 day the lights were extinguished, the leaves again closed. 



237. The period at which the leaves fall off is termed the 



Defoliation* of the plant. About the middle of Autumn, the 



leaves of all annual, and of many perennial plants, begin to 



ose their vigour, change their colour, and at length fall from 



heir stems. 



* From de, signifying to deprive of, and folium, leaf. 



233. What is a necessary food of plants 1 



234. What is meant by the sleep of plants 1 



233. How was Linnaeus led to observe the appearance of plants in 

 the night 1 



236. What experiment was once made with the sensitive plant 7 



237. What is the defoliation of plants'? 



