THE SEASON WHY. 299 



1 The earth is full of the goodness of the Lord." PSALM xxxm. 



1185. Why do ripe fruits taste sweet, and unripe fruits 

 taste sour ? 



Because the juices of the ripe fruit contain a large proportion of 

 sugar, which in the unripe fruit has not been formed. 



1186. Why do some leaves turn yellow ? 



Because they retain an excess of nitrogen. Leaves undergoing 

 decay turn either yellow, red, crimson, or violet. Yellow is due to 

 the excess of nitrogen ; red and crimson to various proportions of 

 oxygen ; violet to a mixture of carbon ; and green to chlorophyl. 



1187. Why do leaves fall off in the autumn ? 



Because they have supplied for a season the natural wants of the 

 tree. Every part has received nutrition through the spring and 

 summer months ; and the wants of the tree being supplied, the 

 chief use of the leaf ceases, and it falls to the ground to decay, and 

 enrich the soil. 



1188. Why do plants suffer from the smoke of cities ? 

 Because the smoke injures the porous structure of the leaves, 



and interferes with their free respiration. 



LESSOIT LXI. 



1189. Wliy are vegetable productions so widely diffused ? 



Because they everywhere form the food of the animal creation, 

 Without them, neither man nor beast could exist. Even the flesh- 

 eating animals are sustained by them, since they live by preying 

 upon the bodies of vegetable -eaters. 



They also enrich and beautify the earth. They present the most 

 charming diversities of proportions and features. From the 

 cowslip, the primrose, and the blue-bell of our childish days, to 

 the broad oak under which we recline, while children gambol round 

 us, they are all beautiful or sublime, and eminently useful in 

 countless ways to man. 



They spread a carpet over the surface of the earth ; they cling to 

 old ruins, and c! ver hard rocks, as though they would hide decay, and 



