404 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



DISCUSSION. 



The investigations so far s'hovv that there are two classes of 

 pollen grains, one requiring only water for germination, the 

 other requiring 'besides water the addition of chemical sub- 

 stances such as acids, sugars and salts. Plants having pollen 

 belonging to the first class have long been known, Mohl (15) 

 having discovered in 1834 that the pollen of the Marina would 

 germinate in water. As investigations go on the list of plants 

 known to have this type of pollen is being rapidly extended. 

 Hansgirg (5) germinated the pollen of Phalaris hracliystachya 

 in water. The pollen of other grasses investigated by him bursts 

 in water. Lidfors (9, 10) germinated the pollen of some species 

 of Rhododendron, Azalea, Erica, Nieiaiiana and Glaucium in 

 distilled water. Moliseh (16) germinated the pollen of Amor- 

 pha fruticosa, CGlutea arhorescens and fifteen other species of 

 plants in saturated air. Jost (7) found that the germination of 

 the pollen of a number of the grasses depended upon a limited 

 water supply, and he was able to germinate the pollen of Ar- 

 rhenatherum elatius, Dactylis glomerata, Bromus mollis, Gly- 

 ceria aquatica, Secale cereale, Zea Mays on starch paste and on 

 parchment paper soaked in water or sugar solutions and prop- 

 erly dried. The pollen of Dactylis germinated when spread on 

 the under surface of the leaves of Limnantheni'Wm nyniphaeoi- 

 des. The pollen of a number of the Compositae and Unibellif- 

 erae Jost germinated on parchment paper only w^hen they were 

 soaked in sugar solutions and properly dried. He did not de- 

 termine whether or not the sugar had any other function than 

 that of controlling the w^ater supply. Martin (13, 14) found 

 the germination of the pollen of Trifolium pratense, Trifolium 

 hyl^ridtmi and MecUcago sativa to depend upon water supply 

 which could be controlled artificially by g-erminating the pollen 

 on animal membranes soaked in water and properly dried. 



Tokugawa (22) found that the pollen of some of the Compo- 

 sitae and UmheUiferae wliich previous investigators failed in 

 germinating, germinated in 25 to 50 per cent sugar solutions 

 and on parchment paper. He concluded that the germination 

 of the pollen of these plants depended only upon water supply. 

 An examination of the stigmas of these plants showed that there 

 was no secretion and that the pollen absorbed the required 

 amount of water from the papilla^, Tokugawa (22) germinated 

 the pollen of Brassica. campestris on the under epidermis of 



