FOOD. 85 



in the soil, tand, being dissolved by the rain, is taken up in a 

 fluid state by the plant as silicate of potash or silicate of 

 soda. The microscope has discovered it in the form of a 

 perfect wall of exquisite masonry of crystals or rapliides, 

 from the Greek, r aphis, a needle. This gives the graceful 

 cane its strength, and enables the slender wheat and barley 

 stems to bear upright heavy heads of grain. 



223. The Dutch rush contains such an amount of these 

 raphides, that before the invention of sand-paper, it was 

 extensively used in the arts for polishing purposes. It is 

 stated on good authority, that reeds, during hurricanes in 

 tropical regions, have been set on fire by the friction of the 

 stems striking and lashing each other, owing to the super- 

 abundance of silica (flint) in them. 



tli. You have been told of the food of plants, how ab- 

 sorbed, and how assimilated. The sap is the medium by 

 which this food is conveyed, in spring-time, through the 

 wood to the remotest branches of the trees, and continues 

 to rise until the leaves fed by it are developed into organs 

 competent to perform the functions of absorption, elabora- 

 tion, and exhalation. Then the sap descends slowly and 

 through another set of vessels to the root again, taking out 

 of the plant all extraneous and useless matter not expelled 

 through the epidermis of the bark and leaf. 



225. It is called, in its first movement, ascending sap or 

 lymph ; when flowing downward, descending sap or juice. 

 The lymph is thin and watery, possessing no peculiar char- 

 acteristics, being perfectly innoxious even in poisonous 

 plants. There is a plant in the Canary Islands whose 

 copious ascending sap is freely drunk by the thirsty trav- 

 eler, while the descending sap of the same plant is a deadly 

 poison. The ascending sap goes up with wonderful force. 



223. What of the Dutch rush ? What of reeds in tropical countries ? 



224. What is the medium through which food is conveyed? 



225. What is the ascending sap called? and through Avhat vessels does it go? 

 "What course does the descending sap take ? and what is it called ? Does lymph 

 possess any peculiar properties ? 



