EXPERIMENTAL EMBRYOLOGY 377 



viable even after 336 days. Air-dry pollen of Coffea stored in the 

 ordinary way loses its germination power within a week, but when 

 kept in a desiccator it retains that power for more than a month 

 (Ferwerda, 1937). Pfeiffer (1944), who made similar experiments 

 with Cinchona, found that 5 to 19 per cent of the pollen retained 

 its viability even after a year's storage in darkness at a temperature 

 of 10°C. and a humidity of 35 to 50 per cent. Nebel (1939) has 

 been able to preserve apple pollen for \y 2 years and sour cherry 

 pollen for 5}i years at a temperature of 2 to 8°C. and a humidity 

 of 50 per cent. Even grass pollen, which is notoriously ephemeral, 

 has been kept alive for 15 to 30 times its natural period of viability. 

 To mention only two examples, the pollen of Saccharum spontaneum 

 (a wild grass used in crosses with sugarcane), spread out on a watch 

 glass in diffused light, remained viable for only about 6 hours; the 

 same pollen stored in vials plugged with cotton and kept at room 

 temperature, for 12 to 24 hours; and the same kept at 7°C, for more 

 than a week (Sartoris, 1942). Similarly, the pollen of Zea mays 

 stored in pollinating bags in direct sunlight at a maximum tempera- 

 ture of 46°C. remained viable for only 3 hours; that stored in shade 

 at a maximum temperature of 30°C, for 30 hours; and the same 

 kept in tassel at a temperature of 4.5°C. and a relative humidity of 

 90 per cent, for 8 to 9 days (Jones and Newell, 1948). 



The data given above indicate that the most important factor in 

 pollen storage is temperature and the next is relative humidity. 

 We know very little about the effect of light; but strong light is 

 undoubtedly harmful, and diffused light or even complete darkness 

 seems to be more conducive to successful storage. 



Equal in importance to the viability of pollen is the receptivity 

 of the stigma, but this is less amenable to control. In many plants 

 the stigma is receptive for only a short period, and if the pollen is 

 not transferred to it at the right time, it fails to germinate, or the 

 germination is so slow that the flower withers and falls off before the 

 pollen tubes can reach the ovules. Attempts to prolong the re- 

 ceptivity of the stigma have usually been unsuccessful, or they are 

 associated with secondary effects which make it difficult to derive 

 much benefit from such prolongation. For instance, although a 

 lowering of the temperature can lengthen the blooming period and 

 also extend the period of receptivity of the stigma to a certain extent, 

 it has an adverse effect on the rate of growth of the pollen tube, so 



