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 l investigator found that no 

 fertilization occurred in some locules, and that the portion of the 

 ovary surrounding these locules 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. 



