(52 H. E. KNowLTon 
dealing with optimum temperatures and influence of stigma. Respiration, 
enzyme activity, moisture content, and carbohydrate reserves were also 
studied, both with fresh and with stored pollen. It was believed that 
these experiments would shed some light on the causes of death. Exten- 
sive storage experiments were made at different conditions of temperature 
and humidity. The influence of reduced pressures was noted and the 
-influence of high and low temperatures on pollen viability was also studied. 
Some pollen was also stored in carbon dioxide and some in oxygen. 
HISTORICAL 
Pollen germination 
Much work has been done by previous investigators on pollen germi- 
nation and on pollen-tube growth in artificial media. Generally, these 
workers have endeavored only to find the best medium for pollen germi- 
nation. It is doubtful whether an artificial medium which in any way 
approximates conditions as found on the stigma and in the style has yet 
been found. The length of the pollen tubes developed on artificial media 
is generally insufficient for the complete penetration of the style. Without 
doubt, however, the physiology of germination is similar under both 
conditions. But in the later stages of growth within the style there is, 
according to East and Park (1918),' a more rapid elongation, while on 
artificial media the growth becomes progressively slower. 
Van Tieghem (1869) seems to be recognized as the first investigator 
to have germinated pollen artificially, altho Von Mohl (1834) had pre- 
viously germinated pollen of Morina in water. Von Mohl noted that. 
in water the pollen tubes did not grow as long as they would need to grow 
under natural conditions. 
Many species of pollen are not sensitive as to the kind or concentration 
of medium. Adams (1916) and Knight (1918) found that apple and other 
fruit pollen germinated over a wide range of concentrations of sugar, 
and even in water alone. A small amount of agar or gelatine added to 
the germinating medium improved germination with some species, 
according to Mangin (1886), Kny (1882), Garino-Canina (1914), and 
Andronescu (9115). 
1Dates in parenthesis refer to Bibliography, p. 789. 
