CH. VIIl] POLLEN TUBES. 217 



into this is stirred a quantity of pollen. Then the stigma 

 and one or two ovules are inserted at various places in the 

 jelly while it is still fluid. Care must be taken to avoid 

 air-bubbles. The slide is then placed in a saturated at- 

 mosphere, preferably in darkness. After a few hours, in 

 warm weather, the pollen tubes will be seen under the 

 microscope directed towards the stigma, the cut-end of the 

 style and the ovules. 



Plantago media is also a good plant for the experiment, 

 and various species of Reseda work well if 1 p.c. of cane- 

 sugar be added to the gelatine. 



The gelatine soon becomes mouldy and fresh material 

 must be made up, or else the old must be kept sterile by 

 repeatedly heating to 80 C. in a water-bath. 



The most perfect demonstration of chemotaxis may be 

 made by mounting Scilla pollen with a stigma in a drop of 

 10 p.c. cane-sugar on a slide, and arranging a damp chamber 

 round it with blotting-paper and a cover-slip. The slide is 

 arranged on the microscope and must not be shaken or 

 moved. After a few hours the cover-slip is taken off and 

 the slide examined. The pollen of Narcissus Tazetta may 

 be used in the same way in 7 p.c. sugar. 



(249 c) Chemotaxis : pollen tubes. 



A preparation of pollen in jelly is made as described in 

 exp. 249 B, except that the ovules and stigmas are omitted 

 and that a cover-glass is placed on the drop. The pollen 

 tubes will be observed to grow away from the edge of the 

 cover-slip towards the centre of the drop, i.e. from places 

 rich in oxygen to places poor in oxygen. Cane-sugar 



