THE HIGHER CRYPTOGAMIA. 



233 



tinctly marked upon the latter in the form of slightly pro- 

 tuberant ridges, and admit of the most accurate measure- 

 ments. Each of the following is the mean of at least five 

 measurements which did not differ from one another by 

 more than half a micro-millimeter.* 



Measurements of the apical cells of Ferns having the ^frond- 

 arrangement. 



This proportion of the base to the sides is that of an 

 equilateral triangle with an apical angle of 69 13' 533", 

 and whose angles at the base are 41 32' 13'4"; angles 

 which very nearly approximate to those of a triangle which 

 is bounded by the chords of two arcs of 138 27' 41-53' 

 (two successive steps of the smaller divergence of the ^ frond- 

 arrangement) and by the line uniting the free terminal 

 points of these chords, which line is the chord of an arc 

 of 83 4' 36' 94", being the difference between the larger 

 and the smaller divergence of the ^ arrangement. The 

 apical angle of such a triangle is 41 32' 18-47"; each of 



* 1 m.m.m. = 9 - 0001 millim. 



