34 



Practical Plant Biology. 



apparatus for microscopic measurement, the Ghost-micrometer. 1 With a 

 mature cell in focus prop a transparent squared screen or even a piece of fine 

 wire gauze against the front of the stage of the microscope with its lower 

 edge resting on the table. The sides of the meshes of the screen should be 

 about 2 mm. apart. Using the flat mirror, adjust the condenser so that the 

 yeast cell and the image of the meshes are simultaneously in focus. Alter 

 the inclination of the screen till the meshes appear perfect squares. Now 

 note how many yeast cells would fit along the sides of the images of the 

 square meshes. The number may be about eight or ten. We already know 

 from the measurements recorded by investigators that the diameter of a yeast 



cell is approximately o'oi mm. Hence we 

 know the size of the image of the meshes 

 in the position where we have propped it ; 

 and whenever we wish to estimate the size 

 of a microscopic object in the field of the 

 microscope, we have only to replace the 

 micrometer in the same position and ob- 

 serve over how many meshes or portions 

 of meshes the object extends. 



The Ghost-micrometer also gives much 

 help in drawing. The relation of objects to 

 one another .and their proportions in size 

 to one another in the squared field pro- 

 duced by the Ghost-micrometer may be 

 readily transferred with accuracy to 

 squared paper. 



More accurate work may be done by 

 calibrating with a slide carrying a milli- 

 meter divided to 100 parts. 



The experiments described in the lecture 

 on invertase should be repeated. Make 

 up a 5 per cent, solution of cane-sugar 

 (sucrose). Pour out a little into a test 

 tube and add an equal volume of Fehling's 

 solution. Boil the mixture. Is a precipi- 

 tate formed ? Now make a mixture of 

 equal volumes of the sugar solution and of 

 a 5 per cent, solution of the invertase 

 preparation described in the lecture. 

 Leave for 10 minutes. Then test some 

 of the mixture with an equal quantity of 

 Fehling's solution. Immediately on boil- 

 ing an orange-red precipitate appears. 



FIG. 6 Microscope with Ghost- 

 micrometer in position. 



This is due to the reducing power of the invert sugar (levulose and dextrose) 

 formed by the inversion of the sucrose. A separate test will show that it 

 is not due to a reaction between the copper solution and the enzyme. 



Now boil some of the enzyme preparation, cool, and add to an equal 

 volume of sucrose solution, leave for 10 minutes as before and again carry 

 out the test with Fehling's solution. No orange-red precipitate appears. 

 The boiling of the enzyme has destroyed its power of inversion. 



1 A good form of this simple measuring apparatus may be obtained from 

 Messrs. Thos, Mason & Co., Opticians, 5 Dame St., Dublin. 



