742 PHYSIOLOGY. 



tough or stringy; and also peculiar secretions, which are either 

 unpleasant to the taste or absolute^^ injurious. The formation of 

 these secretions and woody matter is prevented when light is more 

 or less absent, and hence such then become useful vegetables. 



How such a vast variety of compound substances can be 

 formed in such simply organised bodies as plants are, at present 

 we know but very little. It is to the combined labours of the 

 chemist and physiologist that we must look for the elucidation 

 of this important matter, but it is not our purpose to allude 

 here to the various theories that have been entertained upon 

 their formation and nature, but we must refer the reader who 

 wishes to become acquainted with this subject, to Mulder's 

 "Physiological Chemistry," as translated by Fromberg and 

 Johnston, in which he will find full details. 



5. Effects of Gases generally npon 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 various products and secretions, is absorbed. 

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

 atmosphere composed simply 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 effect of carbonic acid, when in excessive quantities, 

 would seem to be owing to a directly poisonous influence which 

 is then exerted by it upon them. When 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 when 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 atmospheres containing much ammonia, 

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

 action of sulplmrous and hydrochloric acid gases upon plants 

 appears to resemble that of irritants upon animals, for they first 

 exert a local action upon the extremities of the leaves, and then 



