developed in malt wort gelatin and in meat gelatin ; from a technical point of view those developing in malt wort gelatin are of most importance. 2. Qualitative Method of Examining Air in which Petri Dishes are Employed.-Prepare in the usual manner two Petri dishes, one containing wort gelatin, the other meat extract gelatin. Place the dishes where it is desired to examine the air, remove the covers and allow the dishes to remain open for fifteen minutes. Replace the covers, and allow the germs which have fallen on the surface of the gelatin to develop at room temperature in a dark cupboard. Examine the colonies which develop, in the manner described in the previous method (see Klöcker, p. 154). Mould colonies spread more rapidly when the second method is employed than with the first, and are therefore more apt to interfere with the success of the Petri dish method than when the roll culture method is used.1 Hansen's Quantitative Method of Examining Air.-(For description of this process, see Klöcker, p. 150.) Biological Examination of Water for Technical Purposes. It has been pointed out by Hansen that the ordinary biological examination of water conducted with meat extract gelatin is not suitable for technical purposes connected with the 1 The too rapid growth of mould colonies may be controlled by touching them with a small drop of a strong solution of perchloride of mercury in alcohol. fermentation industries, for the nutrient medium employed favours the development of a large number of organisms which are incapable of development in unfermented or fermented wort and so obscures the main point of the examination, which is the recognition of the special organisms which might occasion trouble in technical practice. For this reason it is advisable to use sterilised wort or sterilised beer as nutrient media in the examination of water for technical purposes. (For a description of the method of examination, see Klöcker, p. 145, and also Hansen, Practical Studies in Fermentation, chap. iv., p. 110.) Biological Examination of Water for Hygienic Purposes. It is desirable that the student should have some experience of this process. Examination of Ordinary Tap Water. Requisites: Tubes of gelatin meat extract. Sterilised 1 c.c. pipettes (graduated in tenths). Melt three gelatin tubes, and transfer 0·5 c.c. of the water to one gelatin tube, 03 c.c. to the second and 0.1 c.c. to the third. Mix the water and gelatin in the tubes by swinging them round in the hand rapidly, but be careful to avoid frothing. Pour the gelatin in the three tubes into separate Petri dishes, and after marking each culture with the dilution of water used, allow them to set on a level surface. Incubate the cultures at about 20° for three or four days, and count the colonies formed, expressing the result as so many organisms found in 1 c.c. of the original water taken. The nature of the colonies found, whether they consist of liquefying organisms, mould growths, etc., should be carefully noted. Examination of Polluted Water.-If the water to be tested is polluted with sewage or otherwise contaminated, it will be necessary to use much smaller volumes than those mentioned above for the purpose of inoculation. In order to do this the water must be diluted with sterile water or meat extract. A description of the method usually adopted will be found in any text-book of bacteriology. SECTION IV. THE HOP. THE hop (Humulus lupulus) is a dioecious plant, that is to say, the male and female organs of the plant are contained in separate flowers which are borne by different plants. The female plant alone is cultivated by the hop grower, as its flowers alone develop into the well-known hop-cones. The male hop is usually found as a self-sown plant growing wild in the vicinity of hop gardens, and its sole function so far as the formation of hop-cones is concerned is to fertilise the flowers of the female plant with pollen from the anthers of its male flowers, and so lead to the development of seed in the hop-cone.1 2 Examine a Branchlet of a Hop Plant Bearing Female Flowers. In this state the plant is usually described by the hop grower as being in "burr' (Fig. 3). 1 For a comprehensive account of the hop plant, see Agricultural Botany, by J. Percival, p. 322. 2 Specimens of the hop in flower may be preserved in spirit for the purpose of examination. It will be noticed that the flowers have the appearance of small brushes proceeding from a cone composed of small pointed bracts, or scales. The brush-like appearance is due to long stigmata FIG. 31.-Female Inflorescence of the Hop. proceeding from the ovaries of the flowers which are concealed within the scale-like bracts. Each ovary is surmounted by two stigmata (Fig. 32 c), and has at its base a very small scale-like bracteole (Fig. 32 6). FIG. 32.-Diagram of a Female Flower of the Hop. After the flowering stage is over, the stigmata shrivel, the main axis of the flower lengthens, and the scale-like bracts (Fig. 32 a) and smaller bracte |