298 THE REASON WHY. 



" For the sun is no sooner arisen with a burning heat, but it withereth the grass, 



and the flower thereof falleth, and the grace of the fashion of it perisheth : 



so also shall the rich man fade away in his ways." JAMES i. 



movement of the sap from the root through the stems to the leaves, 

 and downwards from the leaves through the bark to the root. 



1180. Why does the sap of plants thus ascend and 

 descend ? 



Because it conveys upward from the ground some of the matter 

 by which the plant is to be nourished, and which must undergo 

 digestion in the leaves ; and it brings downward from the leaves 

 the matters absorbed, for the nourishment of the plant, and dis- 

 charges through -the root the substances which the plant cannot use. 



The movement of the sap is most active in the spring ; but in 

 the depths of the winter it almost ceases. 



There are other motions of the sap in plants, whioh are called special, in 

 distinction from the ascending and descending of the sap, which is called 

 general, or common to all plants. The special movements of the sap are 

 peculiar to certain plants, in some of which a fluid, full of little green cells, is 

 found to have a rotatory motion ; in other plants, a milky fluid is found to 

 move through particular tissues of the vegetable structure. 



1181. Why are the leaves of plants green ? 



Because they secrete a carbonaceous matter, named chlorophyl, 

 from which they derive their green colour. 



1182. Why are the hearts of cabbages, lettuces, Sfc., of a 

 pale yellow colour ? 



Because the action of light is necessary to the formation of 

 chlorophyl ; and as the leaves are folded upon each other, they 

 exclude the light, and the green matter is not formed. 



1183. Why do leaves turn brown in the autumn ? 



Because, when their power of decomposing the air declines, the 

 oxygen absorbed in the carbonic acid gas, lodges in the leaf, 

 imparting to it a red or brown colour. 



1184. Why do succulent fruits, such as gooseberries, 

 plums, fyc., taste acid ? 



Because, in the formation of juices, a considerable amount of 

 oxygen is absorbed, and the oxygen imparts acidity to the taste. 



