PHYSIOLOGY OF THE LEAF. 83 



subject ; he stated as the result of his investigations, that the 

 opening of the leaf buds of the Birch tree, (Betula,) was the 

 most proper time for the sowing of barley. The Indians of our 

 country had an opinion, that the best time for planting Indian 

 corn, was when the leaves of the White oak, (QUERCUS alba,) 

 first made their appearance ; or according to their expression, 

 are of the size of a squirrel's ears. 



One of the most remarkable phenomena of leaves, is their 

 irritability, or power of contraction, upon coming in contact 

 with other substances. Compound leaves possess this property 

 in the greatest degree ; as the sensitive plant, (MIMOSA sensitiva :) 

 and the American sensitive plant, (CASSIA nictitans :) these 

 plants, if the hand is brought near them, seem agitated as if 

 with fear, but as plants are destitute ^of intelligence, we must 

 attribute this phenomenon to some physical cause, perhaps the 

 warmth of the hand, which produces the contractions, and dila- 

 tations of the leaves. 



The effect of light upon leaves is very apparent, plants being 

 almost uniformly found to present their upper surfaces to the 

 side on which the greatest quantity of light is to be found. It 

 has already been observed that plants throw off oxygen gas ; 

 but for this purpose they require the agency of light. 



Carbonic acid gas is the food of plants; this consists of car- 

 bon 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. 



Many plants close their leaves at a certain period of the day, 

 and open them at another ; almost every garden contains some 

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

 ticularly remarkable in the sensitive plant, and the tamarind 

 tree. This folding up of the leaves at particular periods, has 

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

 apply to plants ; but a celebrated botanist* remarks, " this fold- 

 ing up of the leaves may be as useful to the vegetable constitu- 

 tion, as real sleep is to the animal." Linnaeus was led to ob- 

 serve the appearance of plants in the night, from a circum- 

 stance which occurred in raising the Lotus plant ; he found 

 one morning some very thrifty flowers, but at night they had 

 disappeared ; this excited his attention, 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 man- 

 ner of sleep. He found that some folded their leaves together, 



* Sir J. E. Smith. 



Irritability Effect oflight Sleep of plants. 



