Feb. i6, 1920 Germination of Barley Pollen 531 



humid climate. It would surely burst at night, when the condensation 

 of moisture is heavy. 



RETENTION OF VIABILITY IN THE LABORATORY 



In the laboratory the attempts to keep barley pollen in such a way as 

 to retain its viability met with no better success than in the field. 

 Methods of keeping pollen of a few species of plants have been known 

 for many years. Recently Pfundt (9) has been able to keep the pollen of 

 many plants by regulating the amount of moisture in the air by means 

 of solutions of dilute sulphuric acid of different water tensions. Swingle 

 (id) and others have succeeded in keeping pollen by partial pr complete 

 drying in a vacuum. The laboratory attempts reported here were of 

 three classes: 



(i) Pollen left in the free air at 18° C. for different periods of time 



(2) Pollen kept over sulphuric acid of different strengths. 



(3) Pollen partially or completely dried in vacua at different tem- 

 peratures. 



In all cases the viability of the pollen was ascertained not only by 

 artificial methods of germination but also by testing the germinative 

 power directly upon the barley stigma. It was found advantageous to 

 follow the growth by means of reagents, although the preliminary stages 

 are characteristic and readily seen. Pollen brought in contact with the 

 stigma attaches itself by means of some adhesive substance present, not only 

 on the entire surface of the stigma hairs but also on the pollen grain itself. 

 Normal viable pollen shows a distinct swelling, evidenced by different 

 bulgings of the surface of the grain, which gradually disappear. The 

 swelling is followed by a protrusion of the tube. This protrusion is very 

 short, the tube bending immediately upon the grain, exhibiting a distinct 

 contact tropism. The rapidly growing tube soon enters and disappears 

 within the stigma hairs. The whole process of germination lasts from 

 two to four minutes. To follow and differentiate the tube, the prepara- 

 tions were stained with a dilute solution of methyl-green acetic acid. 

 This stains the pollen grain bluish and the contents of the tube light 

 green. If a dilute solution of Congo red is applied subsequently, the 

 tube is colored red, while the grain, which is unaffected, remains blue. 

 Prolonged staining with Congo red will color the grain violet, the stain 

 being so stable that it persists even after washing with a 5 per cent 

 solution of sodium carbonate. Pollen in various stages of germination 

 is shown in Plate 60. In Plate 60, the tube is still attached to the 

 stigma, but the pollen grain has been loosened so as to make it more 

 easily distinguished. 



POLLEN LEFT IN FREE AIR 



Pollen left exposed to the air for only two minutes loses so much 

 moisture that it becomes shrunken. Shrunken pollen, however, does 

 not mean necessarily a nonviable pollen. Such pollen may germinate, 

 156105°— 20— 3 



