478 AMERICAN JOURNAL OF BOTANY [Vol. 8, 



the hills of Vermont, six miles from a town. When this possible source of 

 error was suspected, ragweed pollen was purposely obtained from Michigan, 

 from two parts of New York state, and from Connecticut. It does not 

 seem probable that the bacteria and molds carried by pollen can be so 

 constant as to cause similar enzyme action in each instance. 



/;. The reactions are too rapid to be due to bacteria. With the in- 

 hibiting action of antiseptics the time required for bacteria to develop in 

 sufficient numbers to produce similar changes would be much longer. All 

 the enzyme reactions recorded have occurred within 24 hours, and several 

 have been almost instantaneous. 



c. Slices of wood in water over night are not in any degree sterile, yet 

 bacteria which have free access do not destroy the middle lamellae, but 

 pollen grains do. Pollen grains taken from unopened anthers and put 

 into sterile Petri dishes are not likely to have peculiar bacteria, absent from 

 the immediate environment. Besides, examination of the pollen contami- 

 nation showed only a few omnipresent common forms of bacteria. 



d. Pollen solutions filtered through a Berkefeld filter gave the enzyme 

 action of diastase on starch, and blood fibrin digestion. 



e. It is probable that the ground pollen added something to the milk 

 which stimulated the growth of bacteria already in the milk, and that it 

 was these which caused coagulation rather than the bacteria introduced by 

 the pollen. The reason for this belief is that in all the plates poured from 

 milk to which pollen had been added Bacillus fluorescens liquefaciens was 

 the dominant type. The plates after standing a few days were a bright 

 apple-green from the fluorescent growth. 



On other plates poured later from the pollen extracts only, not once did 

 this form appear. In the latter it was often not until the third or fourth 

 day that colonies of molds occurred. Doubtless there are resistant forms 

 of spores on the pollen which endure the heat of the autoclave and develop 

 under favorable conditions on the agar plates, but these can hardly account 

 for digestions which occur during twenty-four hours. 



The Chemistry of Pollen 



While many kinds of pollen have been examined for certain special 

 constituents such as starch, nitrogen, phosphoric acid, etc., only eight kinds 

 of pollen, as far as I have been able to ascertain, have been analyzed with 

 any degree of completeness. Czapek (1905) discusses topically the occur- 

 rence and distribution of the principal constituents of plants; if a substance 

 has been reported present in pollen he mentions the fact. These scattered 

 references afford a valuable index to the original literature of the earlier 

 analyses. 



According to Heyl (1919 a, p. 672) the walls of the pollen grain constitute 

 65 percent of the structure. Biourge (1892, p. 75) distinguishes four sub- 

 stances in the wall or envelope of pollen grains: cutin, cellulose, pectic 



