84 VITAL ACTIONS. 



the eartLi and air, but it cannot assimilate it ; its tissue 

 grows, but is not solidified by the incorporation of as- 

 similated matter ; aqueous particles accumulate in the 

 interior, a general yellowness ensues, partly from the 

 want of a sufficient power of decomposing carbonic 

 acid, and partly from inability to decompose the water 

 collected in the interior.* The consequence of this is 

 a want of the means of forming the usual secretions ; 

 flavour, sweetness, nutritive matter, are each dimi- 

 nished ; and the power of flowering and fruiting is 

 lost, probably from the absence of a sufficient secre- 

 tion of organisable matter (85). If the unhealthiness 

 of the plant is not so great as to prevent the produc- 

 tion of flowers, still they may not expand, as often 

 happens to double roses in cold summersf in England; 

 or, if the flowers do unfold, the fertilising power of 



* The cause of the formation of different colours in different plants is 

 too obscure a subject to suit the pui-pose of this work. It is, however, 

 as well to observe that the effect of decomposing carbonic acid and ex- 

 haling oxygen is the production of a green colour, the intensity of which 

 is, in general, in proportion to the decomposing cause, that is to say, to 

 light : but that, if from any circumstances water is not given off, but is 

 retained in the system and allowed to accumulate, the green colour is 

 altered and changes to yellow ; as if the vegetable blue, which must 

 exist in combination with yellow in order to form green, were dis- 

 charged. Such, indeed, is Macquart's explanation of the phenomenon ; 

 and it appears most conformable to theory and fact For a short ex- 

 planation of these and other opinions connected with vegetable colour- 

 ing, see Introduction to Botany^ ed. 8, book ii, chap. xvi. 



f Want of a sufficiently high temperature, and too much water in the 

 soil, seem to be, either together or separately, the cause of the diffi- 

 culty experienced by gardeners in making the Double yellow Rose ex- 

 pand its flowers. 



