756 PHYSIOLOGY. 



and also peculiar secretions, which are either unpleasant to the 

 taste or absolutely injurious. The formation of these secretions 

 and of woody matter is prevented when the access of light is more 

 or less prevented, and the plants then become useful vegetables. 



How such a vast variety of compound substances can be 

 formed in such simply organized bodies as plants is at present 

 almost unknown. It is to the combined labours of the chemist 

 and physiologist that we must look for the elucidation of this 

 important matter, but as it is not our purpose to allude here to 

 the various theories that have been entertained upon their forma- 

 tion and nature, we must refer the student to chemical works 

 for full details upon this subject. It is, however, certain that 

 one result of these chemical processes is the elimination of 

 oxygen, already described. The food of plants is highly oxy- 

 genated as compared with most of the important proximate prin- 

 ciples formed within their leaf-cells, and hence a disengagement 

 of oxygen must occur during their formation. 



5. Effects of Gases generally upon Leaves. — In the last section 

 we have seen, that the ordinary normal constituents of atmo- 

 spheric air, namely, carbonic acid, oxygen, nitrogen, and ammo- 

 nia, in certain proportions, are those which are especially 

 necessary for the due elaboration of the various products and 

 secretions of plants, and these we have now shown are absorbed 

 by the leaves or roots. It is by leaves especially, that carbon, 

 which is so essential to plants, and which enters so largely into 

 the composition of its A-arious products and secretions, is absorbed. 

 It must be understood, however, that plants will not live in an 

 atmosphere composed simplj^ of either carbonic acid, oxygen, or 

 nitrogen ; but that for their proper development, these gases 

 must be mixed in suitable proportions ; for if either of them be 

 in great excess, the plants will either languish or perish, accord- 

 ing to circumstances. Plants will, however, flourish in an 

 atmosphere containing a moderate addition of carbonic acid, 

 even more vigorously than in ordinary atmospheric air ; but if 

 the amount be considerably increased, they will perish. This 

 injurious eifect of carbonic acid, when in excessive quantities, 

 would seem to be owing to a directly poisonous influence. 

 Wlien plants are placed in pure nitrogen or oxygen, or under 

 any other circumstances where they cannot obtain a suitable 

 supply of carbonic acid, they soon decay. 



Whilst the above gases in suitable proportions are necessary 

 to the due performance of the proper functions of plants, other 

 gases wlien mixed in the air in which they are placed, act more 

 or less injuriously upon them. This is more particularly the 

 case with sulphurous acid and hydrochloric acid gases, even in 

 small quantities ; but an atmosphere containing much ammonia, 

 common coal gas, cyanogen, &c., also acts prejudicially. The 

 action of sulphurous and hydrochloric acid gases upon plants 



