OF THE LEAVES. 757 



appears to resemble that of irritants upon animals, they first 

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

 influence is communicated into the deeper tissues, and if the 

 plants be not removed into a purer air, they perish ; but when 

 such gases are not in great quantities, if the plants are speedily 

 removed from their influence, they usually revive, the parts 

 attacked being alone permanently injured. 



While such gases act as irritant poisons upon plants, sul- 

 phuretted hydrogen, carbonic oxide, common coal gas, cyanogen, 

 &c., seem to exert an influence upon plants like that produced by 

 narcotic poisons upon animals, for by their action a general 

 injurious influence is produced on their vitality, and a drooping 

 of the leaves, &c., takes place ; moreover, when such is the case, 

 no after removal into a purer air will cause them to revive. 



As the above gases are constantly present in the air of large 

 towns, and more especially in those where chemical processes on 

 a large scale are going on, we have at once an explanation of the 

 reason why plants submitted to such influences will not thrive. 

 The air of an ordinary sitting room, and especially one where 

 gas is burned, is also rendered more or less unsuitable to the 

 healthy growth of plants, in consequence of the production of 

 injurious gases as well as from the dryness of the atmosphere. 



Wardian Cases. — In order to protect plants from the injurious 

 influences thus exerted upon them by the soot and air of large 

 towns, the late Mr. N. B. Ward, some years since, introduced the 

 plan of growing them under closed glass cases, which has been 

 found to succeed admirably. These cases consist simply of a 

 box or trough in which a suitable soil is placed ; in this the plants 

 are put, and the whole is covered by a closely fitting glass case. 

 It is necessary, at first, to water the plants freely. When plants 

 are grown under such circumstances, upon exposure to light and 

 air, transpiration takes place from their leaves, as under ordinary 

 conditions of growth ; the fluid thus transpired is, however, here 

 condensed upon the surface of the glass case which encloses the 

 plants, and ultimately returns to the soil. It is thus brought 

 into contact again with the roots of the plants, to be again 

 absorbed and exhaled by them ; and these changes are con- 

 tinually repeated, so that the plants are always freely exposed 

 to moisture, and do not require a further supply of water for 

 a considerable period. Those plants, especially, which succeed 

 best in a damp atmosphere, as is commonly the case with Ferns, 

 do exceedingly well in such cases. A very important influence, 

 however, which is exerted by the invention is, the protection of 

 the plants from the immediate contact with the air impregnated 

 with soot and other injurious substances ; for in consequence of 

 the glass cover fitting closely to the trough in which the plants 

 are placed, the external air in its passage has to pass through 

 the very narrow crevices beneath the cover, and in so doing, 



