Fig. 9. Root hairs growing among soil particles.
(Much magnified.)
You can find out some of the importance of these mineral salts in the life of the plant, if you do the following experiment.
Take several seedlings which have already grown enough to have nearly exhausted the supply of food in their cotyledons. These you must grow in jars of pure distilled water, to which you have added certain salts which have been found to be the important ones in the soil water and plant food. By giving the plant nothing but these salts and distilled water you know just what it gets. Distilled water is made by catching and condensing steam, and it has no salts dissolved in it; while ordinary tap water has run off some mountain side or risen in some spring from the rocks, and it has many salts in it already, so that it is useless for this experiment.
Take three big glass jars, each with one litre of distilled water, and label them A, B, and C. Into A put nothing further, into B put the following salts, which have been weighed out carefully either by you or by a chemist:—
| Potassium nitrate | 1 | gramme |
| Calcium sulphate | ½ | „ |
| Sodium chloride | ½ | „ |
| Magnesium sulphate | ½ | „ |
| Calcium phosphate | ½ | „ |
then add to C all these salts, and also one or two drops of a dilute solution of iron chloride.
Into the jars fit corks which are split, with a hole in the centre, and pack a plant into each with the part of the stem between the corks wrapped round with cotton wool (see fig. 10), and so fix the plant that its roots are in the solution and its stem and leaves in the air[2] (see fig. 11). Wrap black cotton or paper round the jars so as to keep the roots dark as they would be in the soil.