OF THE LEAVES. 743 



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 re\'ive, the parts 

 attacked alone being permanently injured. 



"While such gases act as imtant 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 an injurious 

 influence is produced generally on their vitality, and a general 

 di'ooping of the leaves, &c., takes place, and 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 manu- 

 factories are going on, we have at once an explanation of the 

 reason, why plants submitted to such influences Avill not thrive. 

 The air of an ordinary sitting room, and more especially 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. The dryness of the atmosphere, and the con- 

 sequent excessive transpiration from the leaves, is also another 

 cause which prevents plants growing vigorously in such situations. 



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

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

 towns, &c., Mr. N. B. Ward, some years since, introduced a plan 

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

 to succeed admirably. These cases consist simply, of a box or 

 trotigh 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 necessar}^, 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 ; this ultimately returns to the soil, and 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 

 frequently 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. The most im- 

 portant influence, however, which is exerted by such cases 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 to 

 them, has to pass through the very narrow crevices beneath the 



