90 ORGANS AND FUNCTIONS OF PLANTS. 



sucks in nourishment, the pores there may be con- 

 sidered as analogous to the pores of the skin or of a 

 raw surface, in animals, which act similarly, but are 

 not therefore considered to be mouths. 



Hales found that a pear-tree weighing seventy-one 

 pounds, sucked up fifteen pounds of water in six 

 hours ; and branches about an inch in diameter, and 

 five or six feet high, sucked up from fifteen to thirty 

 ounces in twelve hours ; while the same branches, if 

 stript of their leaves, only sucked up one ounce in 

 twelve hours. A plant of mint, whose roots Hales 

 plunged into a bent tube with water, made it fall an 

 inch and a half during the day, but only a quarter of 

 an inch during the night ; while a hop plant sucked 

 up four ounces in twelve hours when in a shady place, 

 and eight ounces in a place more open. 



By watering a plant with coloured fluids sufficiently 

 thin to be taken up by the spongelets, their course 

 has been traced up through the plant, in the form of 

 sap ; and from the composition of this sap, it is obvious 

 that the chief food of plants consists of carbonic acid 

 gas, and azote, often in the form of humic acid ! , dis- 

 solved in water and diluted before it passes into the 

 spongelets, consequently the water holding the most of 

 these gases, which are the real food of plants, must be 

 the best for vegetation. This shows the reason why 

 rain and river water, which have most opportunities 

 of being impregnated with these gases, are better for 

 watering with than the stagnant water of pools or 



(1) See ALPHABET OF SCIENTIFIC GARDENING, p. 12. 



