1913] MARTIN—POLLEN OF TRIFOLIUM PRATENSE 113 
controlled by germinating the pollen on parchment paper soaked 
in distilled water and properly dried on filter paper. The pollen 
of some composites would germinate on the parchment paper only 
after it had been soaked in sugar solution and then suitably dried. 
ELFVING (2) was unable to germinate the pollen of Ornithogalum 
Ecklonii and some species of grasses except on the stigma. Glycerin, 
potassium chlorate, and sodium carbonate added to sugar solutions 
had no effect. Hans Mo iscu (7) found that ©.OI-0.05 per cent 
calcium malate or 0.01 per cent malic acid added to the sugar 
solution would cause the pollen of Azalea and Rhododendron to 
germinate. Saltpeter, asparagin, citric acid, and tartaric acid had 
-a slight stimulative effect. Liprorss (9) increased the percentage 
of germination in some species of Erica and Mensiesia by the 
addition of a small percentage of citric acid. VAN TrecHEM 
(I) obtained better germination in some species by adding 
ammonium bitartrate to the medium. SANDSTEN (16) found that 
tomato pollen required a slightly acid medium. Burcx (12) 
observed that the pollen of certain species of Mussaenda would 
germinate in distilled water, but only when a portion of the stigma 
or levulose was added. Levulose could not be replaced by other 
sugars. TISCHLER (15) was able to germinate the sterile pollen 
common in Solanum rostratum, in some of the Commelinaceae, 
Melastomaceae, Pontederiaceae, Liliaceae, Lythraceae, and in the 
genus Cassia of the Leguminosae, by adding diastase to the sugar 
solution. According to Liprorss (9), the presence of a small 
percentage of calcium or potassium salts or a lack of aeration will 
prevent germination in many species. BuRck (12) found that 
levulose inhibited germination in Pavetfa and Antirrhinum and 
caused bursting in Murraya exotica. The work of Motiscu (6, 7) 
Showed that the direction of pollen tubes in some species is due to 
carbohydrates, and in other species to negative aerotropism. Lip- 
FORSS (10) found that proteins attract the pollen tubes in some 
species. In the species investigated by Kny (3) gravity and light 
had no directive influence on the pollen tubes. Some of the earlier 
botanists, and more recently LINDHARD (17), and the work carried 
on by Pammet and Cor, which is not yet published, have shown 
that the pollen of Trifolium pratense is impotent on its own stigma. 
