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THE AMERICAN NATURALIST [Vol. XL 



dehiscence, were allowed to stand in a solution of Kleinenberg's 

 haematoxylin for twenty-four hours. They were then washed in 

 alcohol and changed very gradually from 96% alcohol to pure 

 clove oil. In the clove oil the pollen sacs were broken up and the 

 pollen grains set free in the oil. The pollen grains were studied 

 by mounting a drop of the oil containing them. It was found that 

 the pollen grains stained very unequally, some not having stained 

 at all, and others having stained so densely that the interior could 

 not be seen. Examination showed that the unstained ones were 

 often the most satisfactory objects for study, so no further efforts 

 at staining were made. This is the method used in demon- 

 strating the interior structure of the pollen grain to large classes, 

 and the author has no claim as the originator of the method. 

 When the grain is thoroughly permeated by the clove oil it becomes 

 so transparent that the interior can be seen very plainly. 



The contracted condition of the protoplasm of the various cells 

 in the interior of the pollen grain made the outlines of these cells 

 very plain, and their number easy to determine with certainty in a 

 large series of the grains examined. Thus the crudeness of the 

 method of killing the material proved a decided advantage in the 

 study of it. Another advantage appeared in mounting the pollen 

 grains in clove oil, in that the same individual pollen grain could 

 be studied from different points of view on causing it to roll over 

 by means of pressure on the cover glass. Two views of the same 

 pollen grain are shown in Figs. 11 and 12, also in Figs. 13 and 

 14 (PI. 1). 



Historical 



The older literature concerning the pollen grain has reference 

 to the external characters and structure, and for the internal struc- 

 ture there is no need to go farther back than 1834, when von Mohl 

 published his work, Ueher den Ban und die Formen der Pollen- 

 korner ('34). In this article von Mohl cites the older literature 

 on the subject, and figures pollen grains of a few gymnosperms. 

 His figures, however, do not show internal structure, and only in 

 the work of Fritzsche (1836), flo we first find figures which show 

 something of the internal structure of the pollen of gymnosperms 



