40 THE FOOD OF PLANTS CHAP. 



The Seed obtains Pood from the Air 



Experiment 25 



Pour a little distilled water into a saucer. 

 Weigh out one gramme of mustard or cress seed, 

 and scatter it on some water placed in the saucer. 

 Place the saucer in the window, and keep it well 

 .supplied with water. Let the water used be distilled, 

 so that it may contain no food for the young plant. 



The seeds will soon begin to sprout and grow. 

 When they have grown to a full crop remove the 

 young plants, place them in the platinum tray, add 

 the water the saucer contains, and then place them 

 in the water oven, dry them completely, so as to 

 remove the water they contain, and then weigh. 



The dried plants will weigh more than the seed, 

 showing that the seeds cannot have supplied 

 all of the material used in the growth of the 

 plant. 



The seeds themselves contain a little water, losing, 

 on drying, about 15 per cent of their weight. Con- 

 sequently, in order to compare exactly the weight of 

 the seed with the weight of the plants, this should 

 be allowed for. 



The Food Obtained from the Air is Charcoal 



Now let us go a step farther, and try and deter- 

 mine the nature of the food obtained by the plant 

 from the unknown source. 



