10 THE NATURE AND WOBK OF PLANTS 



when we have finished we have nothing but a pile 

 of rubbish and dust. 



Then, again, we cannot actually construct proto- 

 plasm with our own hands; but we may ascertain 

 the substances which should be given to the proto- 

 plasm of a plant in the form of food, in order that it 

 should be able to grow or add to its bulk. While 

 both of these methods are subject to so many errors 

 that they are but of little use separately, yet both 

 together give a fair knowledge of the composition of 

 the bodies of plants. 



8. Water, charcoal, and ash. — Place enough 

 freshly gathered leaves or stems to make about half 

 a pound in a small tin dish, and set on the pan of a 

 balance. Weigh carefully. Now place the pan on 

 the top of a stove for two or three hours, or in the 

 hot sunlight for twice that time. Weigh again. 

 How much weight has been lost ? Set fire to the 

 dried material and attend to it until it is completely 

 burned, being careful that none of the ash is allowed 

 to escape from the dish. Now weigh again. How 

 much weight has been lost this time ? Clean out 

 the pan and weigh. Subtract the weight of the dish 

 from the figures obtained at each weighing and you 



