1837*3 Gn tne Growth of Plants without exposure to open Air. 341 



contrary, I wished to grow Cacti, they were planted in a mixture of 

 loam and sand, suspended ^"rom the roof of the case, and fully exposed 

 to solar heat. Upon this part of the subject I need not, however, dilate 

 any further, and will therefore confine myself to the results obtained. 



1st, That the depressing influence of the air of large towns upon 

 vegetation depends almost entirely upon the fuliginous matter with 

 which such an atmosphere is impregnated, and which produces the 

 same effect upon the leaves of plants as upon the lungs of animals. 



2ndly, That, owing to the quiet state of the atmosphere surrounding 

 the. plants in my inclosed cases , the plants, like human beings, will 

 bear extremes of heat and of cold, which under ordinary circumstances 

 would be fatal to them. It is well known, from the experiments of 

 Sir C. Blagden, and others, that man will bear great degrees of heat 

 with impunity, provided the atmosphere be undisturbed, and it is equal- 

 ly a matter of fact, that the extremest cold of the Arctic Regions pro- 

 duces no bad effect, when the air is quite still. Mr. King, who has 

 recently returned from Captain Back's Expedition, informed me that 

 the greatest degree of cold he experienced was nearly 70 deg. below 

 zero ; that no inconvenience was felt at that low temperature, owing to 

 the perfectly calm state of the air ; but that if the wind arose, although 

 the thermometer would likewise rapidly rise with the wind> the cold 

 then became insupportable. 



These facts I have proved in the one case, by the exposure to sun of 

 Hymencphylhim and Trichomanes ; and in the other by growing with- 

 out heat Aspidium molle, Phoenix dactylifera, Rhapis flabelliformis i DeH-> 

 drobium pulchellum, Mantmillaria tenuis, &c. &c. 



3rdly, That owing to the prevention of the escape of the moisture 

 contained within the cases, plants will grow for many months, and 

 even for years, without requiring fresh supplies of water. Thus, in 

 the first experiment, the Poa and Nephr odium grew for four years* 

 without one drop of water having been given to them during that 

 period, and would, I believe, have grown as many more^ had they not 

 accidentally perished in consequence of the rusting of the tin lid co- 

 vering the bottle, and the admission of rain-water. 



4thly, That the degree of development to which the plants attain, 

 depend mainly, ceteris paribus, upon the volume of air contained with- 

 in the case, and upon the quantity of light and solar heat received by 

 the plants. Thus to revert to the first experiment. The Poa and 

 JVephrodium, being contained within a small bottle — the one flowered 

 but once during its confinement, while the other did not produce any 

 capsules. Both Ferns and Grasses, in my larger cases, flower and fruit 

 well. Phcenogamous plants, such for instance, as Ipomcsa Quamoclit 

 and coccinea, will not flower in a case exposed to the north, while in 



