OMPHALIA FLAVIDA 423 



The Detachment of a Gemma from its Pedicel. — Ripe gemmae 

 could be readily blown away from their pedicels as follows. A 

 hollow glass tube was drawn out to a slight nozzle at one end, the 

 nozzle was then placed near some gemmifers growing on a Bryo- 

 phyllum leaf, and then air was blown through the tube from the 

 mouth. The gemmae upon which the blast of air impinged were 

 very easily blown away from their pedicels to some distance. There 

 can be no doubt that a slight wind would also have carried off these 

 gemmae. 



A test-tube containing a malt-agar slope which bore gemmifers 

 was held horizontally with the gemmifers looking downwards and 

 was then tapped. Several of the gemmae came off their pedicels 

 and settled on the side of the tube below, others remained on their 

 pedicels. This experiment affords further evidence that ripe gemmae 

 can be detached from their pedicels without much difficulty by 

 mechanical means. 



The gemmae never droj) off their pedicels sjiontaneously and, no 

 doubt, under natural conditions in the open, they remain in situ 

 until the wind is sufficiently strong to bear them away. This 

 resistance to detachment except under conditions favourable for 

 dispersion must be of no small value to a fungus which is a parasite 

 on leaves. It finds its parallel in the fruits of the Manitoban Maple, 

 Acer Negundo, which in Manitoba hang on the trees through the 

 winter until some violent gale in March or April tears them from 

 the branches and scatters them far and wide. 



The mature gemma of a gemmifer situated on the top of a leaf- 

 spot, owing to the basal and the sigmoid curvatures of the pedicel, 

 comes to have its median axis disposed more or less parallel to the 

 leaf surface (Fig. 213, C). This orientation of a gemma, as we have 

 seen, is due to the reaction of the pedicel to certain stimuli. The 

 basal curvature of the pedicel of a gemmifer in a group of gemmifers 

 serves to remove the gemma from neighbouring gemmae, and the 

 sigmoid curvature of the pedicel resulting in the turning downwards 

 of the gemma seems to be significant merely as an incidental factor 

 in the mechanism of abscission. On the other hand, it may be that 

 the final orientation of the gemma contributes something to the ease 

 of detachment of the gemma by the wind. The wind sweeping 



