56 Practical Plant Biology. 



may be found in the cell. They appear to be formed by the 

 contraction of the protoplasm qf the cell from the original wall 

 and round them a second cell-wall is secreted. They are spores. 

 Observation shows that they are often formed when a culture 

 is drying up, or when waste products are accumulating in it. 

 When transferred to a new culture-medium, or when water is 

 added to the desiccated material in which the spores find them- 

 selves, these spores germinate by the solution or bursting of the 

 cell-wall and division of the protoplasm. It has been found ex- 

 perimentally that these spores are much more resistant to adverse 

 external conditions than the ordinary vegetative or growing cells. 

 Thus, while, in many instances, the latter are killed by desiccation, 

 spores withstand complete desiccation for months or even years ; 

 also, while the temperature of 70 C. is usually fatal to vegetative 

 cells, spores may resist 100 C. or even 110 C. (and if thoroughly 

 dried as much as 130 C.) and yet be capable of germination. 



PRACTICAL WORK. 



Mount a drop of the cloudy salt water which has lain in contact with a 

 piece of red sea-weed for a few days. 



With the high power study some of the innumerable specks in the field. 

 Observe both the motionless and the motile ones. Find cocci, bacilli, 

 comma bacilli, vibriones, spirilla, spirochaetes, and leptothrix. 



Dip a glass rod or needle into the culture and smear a little of the liquid 

 on a clean cover-glass, spreading the liquid uniformly, leaving very little on 

 the cover. Set it apart till quite dry. Drying may be hastened by gentle 

 warming over a flame. When quite dry take the cover in a forceps and, 

 holding the smeared surface uppermost, pass the cover two or three times 

 through a Bunsen flame. This kills the bacterial cells and coagulates the 

 colloids on the outside of the cells, fixing them to the glass. Now place a 

 drop of gentian violet solution on the smear and allow the stain to act 

 for 5 minutes. At the end of this time wash the smear thoroughly under 

 a tap, shake off as much water as possible, and wash with a fine jet of 

 methylated spirit. Again shake off as much as possible and leave the smear 

 to dry thoroughly. When quite dry add a drop oi balsam, invert, and place 

 on a slide. 



The different forms present in the culture will be fixed, stained, and pre- 

 served in this smear-preparation. 



It is wise to make a smear-preparation every day after starting the de- 

 caying sea-weed. On a certain day the variety of forms will be at its 

 maximum. Previously to that, and subsequently, the less common forms 

 such as spirilla may be hard to find. 



Careful examination of these permanent preparations should be made with 

 the high power and the various forms observed should be recorded in sketches. 

 Particular care must be taken not to let balsam come into contact with the 

 high power. Should the front lens, by any accident, become smeared with it, 

 wipe the lens as clean as possible with a clean piece of linen or cotton, and 

 then polish it with a fresh piece moistened with spirit. 



