g6 Practical Plant Biology. 



supplies of organic nitrogenous substances. It cannot make use 

 of nitrates for this purpose. 



Unlike yeast it normally requires free oxygen for its respiration. 

 It is, however, capable of life in a submerged condition. The 

 supply of free oxygen is then limited and in response to this 

 changed condition the hyphse undergo transverse division. The 

 portions of the hyphae so formed each gives rise to a rounded cell 

 not unlike that of yeast. These cells when introduced into a 

 sugar solution bud and behave very like yeast-cells. They change 

 the sugar of the solution into alcohol and carbon dioxide and 

 thus show that, in this form at least, Mucor is able to subsist by 

 means of anaerobic respiration. 



PRACTICAL WORK. 



Examine a growth of Mucor on moist bread with a lens. Make out the 

 creeping hyphae and the vertical branches with mature and immature 

 sporangia. Cultures for microscopic examination may be made of the 

 "spontaneous growth" on bread as follows: Draw a sterilised needle 

 through the vertical branches, not letting it touch the surface of the bread, 

 and with it innoculate a test-tube containing bouillon gelatine. Stir the 

 liquid gelatine well with the needle to distribute the spores. With a sterile 

 glass-rod put drops of the gelatine while still liquid on to sterile micro-slides 

 which should be set aside and kept moist under a bell-glass. The slides may 

 be taken out at intervals and examined microscopically. If it is intended to 

 use a high objective in the examination the preparation should be covered. 

 Before covering a drop of alcohol is added to prevent the inclusion of air 

 bubbles. The cover-glass is then laid on the preparation, and the slide is 

 gently warmed over a flame so that the cover-glass settles into position by 

 the melting of the gelatine. 



In these preparations the early stages of germination should be studied 

 and the growth and structure of the mycelium. Beautiful preparations may 

 be made by first fixing the growing mycelium with iodine vapour and then 

 staining with Delafield's haematoxylin. This may be done by inverting the 

 slide and laying it across the neck of a bottle containing iodine crystals, so 

 that the preparation hangs into the neck of the bottle. Fixing is accom- 

 plished in a few minutes and then a drop of Delafield's haematoxylin is added 

 to the gelatine and the slide returned to the moist chamber for 30 minutes 

 or an hour. At the end of this time the stain should be washed away, the 

 preparation covered and warmed gently. This preparation will keep for 

 several days and shows the cytoplasm a pale violet while the nuclei appear 

 as dark purple globules. 



The vertical branches should be examined. In covered preparations they 

 will of course be pressed down on the gelatine. The sporangia will be seen 

 in different stages of development with the transverse septum at first flat 

 and later bulged into the sporangium and forming the columella. The 

 differentiation of the spores may be traced and the changes which convert 

 the wall of the sporangium from a smooth, colourless membrane to a thick 

 greyish wall studded closely with tiny crystals. 



The diameter of the thickest hyphas and the size of the spores and spor- 

 angia should be estimated by means of the Ghost-micrometer. 



Spores sown in gelatine bouillon produce plants which bear ripe sporangia 

 in two days if the cultures are kept at ordinary room temperature. 



