19* PHYSIOLOGICAL BOTANY. PART 11. 



remains any carbonaceous materials in the sap. This 

 may assist us in explaining an interesting fact described 

 in the " Gardener's Magazine," vol. x. p. 208. It is 

 there stated that many plants, especially ferns, have 

 been readily grown in the smoky atmosphere of Lon- 

 don, by placing them in boxes furnished with glass 

 coverings hermetically sealed. In this state they have 

 lived and increased in size during several years, without 

 any immediate communication with the atmosphere. 

 The same mode of treatment has been successfully 

 practised in transporting plants during a long voyage, 

 the influence of the sea breeze charged with saline par- 

 ticles forming the greatest obstacle to their safe con- 

 veyance. When performing experiments to ascertain the 

 decomposition of carbonic acid by the process of respir- 

 ation, great precaution is necessary to ensure accurate 

 results. The plants being placed under conditions 

 which are not strictly natural, are soon apt to become 

 sickly and exhibit a tendency to decompose. When 

 this is the case the formation of hydrogen, water, and 

 other substances takes place, and vitiates the results. 

 Those who are anxious to pursue these researches in 

 further detail may peruse the admirable treatises of 

 De Saussure and Ellis; where they will find a multitude 

 of experiments recorded and a patience of investigation 

 exhibited, which has been rarely surpassed by the la- 

 bours of other philosophers. 



(181.) Vegetable Colours. Not only the green colour 

 of those parts which decompose carbonic acid, but all 

 the various colours of plants, depend upon the presence 

 of minute grains of matter contained in the vesicles 

 of the cellular tissue. The grains which give the 

 green tinge to the leaf are termed " chromule," and 

 it is probable that all the others are only modifications 

 of the same substance. From observations made upon 

 the leaf at different seasons of the year, it appears that 

 towards autumn this organ ceases to give out oxygen by 

 day though it continues to imbibe it by night; and 

 hence it seems highly probable that the chromule passes 



