CHAPTER XIV 



THE SIZE OF SPORES— POYNTING'S PLATE MICROMETER 



Each species produces spores of a definite shape and size. The 

 spores vary in size about a mean, doubtless in accordance with the 

 now well-known laws of continuous variation. The variations as a 

 rule are within fairly restricted limits, so that fungus spores, when 

 observed with a microscope, appear to resemble one another very 

 much as do eggs laid by a fowl. By measuring the diameters of 

 twenty-five spores of any fruit-body, one can obtain an average 

 size which is correct to within a very small percentage of the 

 real average for all the spores. It must not be assumed, however, 

 that all the individual fruit-bodies of a species have spores of 

 the same average size. Thus, for instance, three specimens of 

 Amanito'psis vaginata, obtained from the same wood on differ- 

 ent days, possessed spores with an average diameter of 11*65 n, 

 10-87 fi, and 10-19 p (Fig. 55, B, C, D, p. 162). It is not sur- 

 prising that the spore sizes for species as given by systematists 

 often disagree. 



For the purpose of measuring spores rapidly and accurately I 

 have made use of Poynting's Plate Micrometer, 1 a simple and exact 

 piece of apparatus which should come into general use in all inves- 

 tigations where it is necessary to measure the sizes of numerous 

 small bodies. Since it seems that I am the first to apply the Plate 



1 The Plate Micrometer with which I worked was Professor Poynting's 

 original instrument. My thanks are due to him for kindly permitting me to 

 use it in his laboratory. It was exhibited at an Optical Convention held in 

 London four years ago. In the Proceedings of the Optical Convention, No. 1, Lon- 

 don, 1905, a one-page account of the principle of the micrometer is given, but this 

 would be of little use to any one wishing to understand how the measurements 

 of spore dimensions were made. Professor Poynting has informed me that he 

 has not yet published an adequate description of the Plate Micrometer as applied 

 to the microscope, but he has consented to my attempting to show how the 

 instrument may be used in practice. 



