GROWTH OF PLANTS IN WAED'S CASES. 161 



light. In these circumstances they will continue to thrive for a long 

 time, even for years, without any fresh supply of moisture or any 

 direct exposure to the air. These Cases are well fitted for rooms 

 where the dryness of the atmosphere interferes with the vigour of 

 plants, by causing greater exhalation than can be compensated by the 

 absorption of moisture by the roots. Some plants, as Perns, requiring 

 a humid atmosphere, thrive well in such Oases. 



But it is not merely as objects of luxury and curiosity that these 

 Cases deserve notice. They supply an important means of transport- 

 ing plants, in a living state, to and from foreign climates ; and they 

 are in constant use for that purpose. Plants have thus been brought 

 to this country which could not have retained their vitality in the 

 form of seed, and which would have been destroyed by exposure to 

 the searbreeze and to the vicissitudes of climate experienced during 

 their transport. Plants of Musa Cavendishii have been thus intro- 

 duced into the South Sea Islands, and Tea, Ipecacuan, and Cinchona 

 into our Indian possessions. The stillness of the atmosphere in the 

 Case contributes materially to prevent injurious consequences. In 

 June 1833, Mr. Ward filled two Cases with Ferns,' Grasses, etc., and 

 sent them from Britain to Sydney, where they arrived in January 1834. 

 The plants were taken out in good condition, and the Cases were re- 

 filled at Sydney, in February 1834, the thermometer then being 

 between 90° and 100° Fahrenheit. In their passage to England they 

 encountered very varying temperatures. The thermometer fell to 20° 

 on rounding Cape Horn, and the decks were covered a foot with snow. 

 In crossing the line the thermometer rose to 120°, and fell to 40° 

 on their^arrival in the British Channel in the beginning of November, 

 eight months after they had been enclosed. The plants were not 

 once watered during the voyage, and received no protection by day 

 or by night, nevertheless they reached London in a healthy and 

 vigorous condition. 



It is a mistake to suppose that the air in the Cases remains un- 

 changed. They are not hermetically sealed ; and by the law of diffu- 

 sion of gases there is a constant although gradual mixture of the 

 external air with that inside, free however from many impurities. 

 Plants wiU continue to grow for a long time, even in Cases hermeti- 

 cally sealed, if supplied at first with abundance of good soil and water. 

 By the united action of the plant and light, the air undergoes constant 

 changes, and thus continues fit for vegetable life. 



4. — Products and Secretions of Plants. 



The sap in its progress through the cells and vessels, and especi- 

 ally in its passage through the leaves, is converted into organisable 

 products, from which the vegetable tissues are nourished and the 



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