THE BBASON WHY. 3J9 



" 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 animalcules. 



1263. Why 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 

 tolid rod, containing the same amount of material. 



1267. Why do the stalks of plants lecoine 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 un's rays, and so carry on the function of 



