THE BEASON WHY. 319 



1 For in this mountain shall the hand of the Lord rest, and Moab shall be trodden 



down under him, even as straw is trodden down for the dunghill." 



ISAIAH xxv. 



that are very light, and which, but for this beautiful provision, would 

 be exceedingly perishable. 



1262. Why is guano a productive manure ? 



Because it contains, with other suitable elements, an abundance 

 of the silicons skeletons of animalcule. 



1263. W hy does a wheat-crop greatly exhaust the soil ? 



Because, as well as the carbon, and the salts, which form the 

 straw and the grain, it draws off from the soil a great amount of 

 silica. 



1264. Why is straw frequently used as a manure? 

 Because it gives back, with other substances, a considerable 



proportion of silica, in that form which adapts it to the use of the 

 succeeding crop. 



1265. Why is the structure of herbaceous plants less 

 consolidated than that of woody plants ? 



Because, for the most part, herbaceous plants last only a single 

 year ; they, therefore, do not require the enduring qualities of plants 

 that have to sustain the influences of the elements for a succession 

 of seasons. 



1266. Why are the stalks of plants of light structure 

 generally cylindrical ? 



Because the cylindrical form is stronger than any other; a 

 hollow cylinder, with moderately thick walls, is stronger than a 

 solid rod, containing the same amount of material. 



1267. Why do the stalks of plants become hollow ? 

 Because the parallel and perpendicular fibres of the stalk are 



developed more rapidly than the horizontal. The growth of the 

 plant, therefore, consists of a kind of divergence from the centre. 



1268. Why are the stomata, or pores of leaves, generally 

 placed on their under surface ? 



Because, being placed on the under surface, they are shaded 

 from the action of the sun's rays, and so carry on the Function of 



