In the Rothamsted experiments no attempt is made to carry on the cultures under sterile conditions. Bottles of 600 c.c. capacity are used, after being thoroughly cleaned by prolonged boiling (about four hours) followed by washing and rinsing. The bottles are filled with nutritive solution and the appropriate dose of poison, carefully labelled and covered with thick brown paper coats to exclude the light from the roots and to prevent the growth of unicellular green algae. The corks to fit the bottles are either used brand new or, if old, are sterilised in the autoclave to avoid any germ contamination from previous experiments. Lack of care in this respect leads to diseased conditions due to the growth of fungi and harmful bacteria. Two holes are bored in each cork, one to admit air, the other to hold the plant, and the cork is cut into two pieces through the latter hole. The seeds of the experimental plants are “graded,” weighed so that they only vary within certain limits, e.g. barley may be ·05–·06 gm., peas ·3–·35 gm., buckwheat ·02–·03 gm. In this way a more uniform crop is obtained. Great care is needed in selecting the seeds, the purest strain possible being obtained in each case. With barley it has always proved possible to get a pure pedigree strain, originally raised from a single ear. In this way much of the difficulty due to the great individuality of the plants is overcome, though that is a factor that must always be recognised and reckoned with. The seeds are sown in damp sawdust--clean deal sawdust, sifted and mixed up with water into a nice crumbly mass--and as soon as they have germinated and the plantlets are big enough to handle they are put into the culture solutions. Barley plants are inserted in the corks with the aid of a little cotton wool (non-absorbent) to support them, care being taken to keep the seed above the level of the water, though it is below the cork. With peas it is impossible to get a satisfactory crop if the seed is below the cork, as the plant is very prone to bacterial and fungal infection in its early stages, and damp cotyledons are fatal for this reason. Consequently the mouths of the bottles are covered with stout cartridge paper, the pea root being inserted through a hole in the paper, so that the root is in the liquid while the cotyledons rest on the surface. As soon as sufficient growth has been made the papers are replaced by corks, the remnants of the seeds still being kept on top in the air. Other plants are treated according to their individual needs and mode of germination (Fig. 1). [Illustration: Fig. 1. Diagrammatic sketches showing methods of setting up water cultures. ~A.~ _a._ Seedling of cereal. _b._ Cork bored with two holes, and cut into two pieces through one hole. _c._ Food solution. ~B.~ _a._ Pea seedling. _b._ Paper shield which supports the seedling. _c._ Brown paper cover over bottle of food solution.]