94 LEAVES SOMETIMES EBUT CHLaRlNE. 



become pale and sickly, and refuse lo perform their thost important 

 chemical functions when excluded from the light. The bloom disap- 

 pears from the human cheek, the hody wastes aw»y, and the spirit 

 sinks, when the unhappy prisoner is debarred from the sight of the blessed 

 sun. In his system, too, the presence of light is necessary to the perfor- 

 mance of those chemical functions on which the healthy condition of tlie 

 vital fluids depends. 



The processes by which oxygen and carbonic acid are respectively 

 evolved in plants have been likened by physiologists to the respiration 

 and digestion of animals. It is supposed that when plants respire they 

 give off carbonic acid as animals do, and that when they digest they 

 evolve oxygen. Respiration also, it is said, proceeds at all times, diges- 

 tion only in the light of the sun. Though these views are confessedly 

 conjectural, they are founded upon striking analogies, and may reason- 

 ably be entertained as matters of opinion. 



6°. Other species of decomposition also, besides that o^ de-oxidization^ 

 go on in the leaf, or are there made manifest. Thus when plants grow 

 in a soil containing much common salt (chloride of sodium) or other 

 chlorides, they have been observed by Sprengel and Meyen to evolve 

 chloride* gas from their leaves. This takes place, however, more dur- 

 ing the night than during the day. Some plants also give offamn^ionia, 

 (Lecture IV., p. 70,) while others (crucifera)), according to Dr. Daube- 

 ny, [in his Three Lectures on Agriculture, p. 59,] emit from their leaves 

 pure nitrogen gas. 



The evolution of chlorine implies the previous decomposition of the 

 chlorides, which have been absorbed from the soil; while that of nitro- 

 gen may be due to the decomposition of ammonia, of nitric acid, or 

 of some other compound containing nitrogen, which has entered into the 

 circulation by the roots. The exact mode and nature of the decompo- 

 sition of these substances, and the purposes served by them in the vegeta- 

 ble economy, will come under our consideration in a subsetjuent lecture. 



The leaf has been described (p. 76) as an expansion of tlie bark. 

 It consists internally of twi layers of veins or vascular fibres laid one 

 over the other, the upper connected with the wood — the lower with the 

 inner bark. It is covered on both sides by a thin membrane (epider- 

 mis), the expansion of the outer bark. This ihin inembane is studded 

 with numerous small pores or mouths (stomata), which vary in size and 

 in number with the nature of the plant, and with the circumstances in 

 which it is intended to grow. It is from the pores in the upper part of 

 the leaf that substances are supposed to be exhaled, while every thing 

 that is inhaled enters by those whica are observed in the under side of 

 the leaf.f This opinion, however, is not universally received, it being 

 admitted by some that the power both of absorbing and of emitting 

 may be possessed by the under surface of the leaf. 



7°. We have seen that the chief su])ply of the fluids which constitute 



* Chlorine is a gas of a greenish yellow colour, having an unpleasant taste and a suffocating 

 odour. Wiien it combines with other substances it forms chlorides. It exists in, and im- 

 parts its smell to, chloride of lime, which is employed for disinlecting purposes, and it 

 forms upwards of half the weight of common salt. 



t This is illustrated by the action of a cabbage leaf on a wound. If the upper side be ap- 

 plied, the sore is protected and quickly heals, while the under side draws it and produces  

 constant discharge. 



