=54. Absorption by land plants.=—_Water cultures._—In connection with our inquiry as to how land plants obtain their water, it will be convenient to prepare some water cultures to illustrate this and which can also be used later in our study of nutrition (Chapter IX). [Illustration: Fig. 39. Young frond of wolffia growing out of older one.] [Illustration: Fig. 40. Young frond of wolffia separating from older one.] [Illustration: Fig. 41. Another species of wolffia, the two fronds still connected.] Chemical analysis shows that certain mineral substances are common constituents of plants. By growing plants in different solutions of these various substances it has been possible to determine what ones are necessary constituents of plant food. While the proportion of the mineral elements which enter into the composition of plant food may vary considerably within certain limits, the concentration of the solutions should not exceed certain limits. A very useful solution is one recommended by Sachs, and is as follows: =55. Formula for water cultures=: Water 1000 cc. Potassium nitrate 0.5 gr. Sodium chloride 0.5 “ Calcium sulphate 0.5 “ Magnesium sulphate 0.5 “ Calcium phosphate 0.5 “ The calcium phosphate is only partly soluble. The solution which is not in use should be kept in a dark cool place to prevent the growth of minute algæ. =56.= Several different plants are useful for experiments in water cultures, as peas, corn, beans, buckwheat, etc. The seeds of these plants may be germinated, after soaking them for several hours in warm water, by placing them between the folds of wet paper on shallow trays, or in the folds of wet cloth. The seeds should not be kept immersed in water after they have imbibed enough to thoroughly soak and swell them. At the same time that the seeds are placed in damp paper or cloth for germination, one lot of the soaked seeds should be planted in good soil and kept under the same temperature conditions, for control. When the plants have germinated one series should be grown in distilled water, which possesses no plant food; another in the nutrient solution, and still another in the nutrient solution to which has been added a few drops of a solution of iron chloride or ferrous sulphate. There would then be four series of cultures which should be carried out with the same kind of seed in each series so that the comparisons can be made on the same species under the different conditions. The series should be numbered and recorded as follows: No. 1, soil. No. 2, distilled water. No. 3, nutrient solution. No. 4, nutrient solution with a few drops of iron solution added. [Illustration: Fig. 42. Culture cylinder to show position of corn seedling (Hansen).]