HOW POLLEN IS AFFECTED BY EXTERNAL FACTORS 49 
the mature fruit some undeveloped ovules, which due to the lack 
of fertilization did not become seeds. Although much of the vari- 
ation that occurs in the number of seeds in many of the fruits is 
due to the failure of the pollen to function properly on the stigma 
or to the insufficient nourishment of the ovules, much of the vari- 
ation can be attributed to insufficient pollination. 
There is good evidence that the imperfect development of 
fruit is due in some cases to insufficient pollination. By polli- 
nating the stigmas of Tomatoes in such a way that portions of 
the stigmas received no pollen, one! investigator found that no 
fertilization occurred in some locules, and that the portion of the 
ovary surrounding these Jocules developed much less than those 
portions of the ovary surrounding those locules in which fertili- 
zation occurred, thus causing one-sided fruits. 
How Pollen is Affected by External Factors. — Pollen is not 
so specially prepared as seeds are to endure extreme conditions 
during transportation. During transportation and while on the 
stigma, pollen may be either killed or rendered functionless 
by extremes of temperature and moisture. The pollen of most 
plants is so sensitive to dryness that an exposure to the ordinary 
dryness of the air cannot be endured more than a few days and 
in many cases only a few hours. 
In the storage of pollen, which is sometimes necessary in experi- 
mental work, the main caution is to store the pollen where it 
will not be dried out too much by evaporation, although the pol- 
len must be kept dry enough that it will not mold. It has been 
found that Plum and Apple pollen can be kept alive much longer 
when stored in closed chambers where there is less drying than 
in laboratory air. One investigator has reported that Corn pol- 
len will die in two or three hours when exposed to the air of the 
laboratory or living room, but will live two days when stored 
in a moist chamber. Some investigators think that hot dry 
weather during the pollination of fruit trees may affect the setting 
of fruit by destroying some of the pollen. 
The pollen of some plants, as in case of Red Clover and Alfalfa, 
absorbs water so rapidly that it is destroyed by bursting when 
immersed in water or stored in a saturated air. Consequently 
these plants are not successfully pollinated when they are wet 
1 Pollination and Reproduction of Lycopersicum esculentum (Tomato). 
Minnesota Botanical Studies, p. 636, Nov. 30, 1896. 
