V. 



On the Dispersal of the Nutlets in certain 



Labiates/ 



ONE of the commonest methods of seed dispersal in plants with 

 capsular fruits is what is sometimes called the " censer 

 mechanism." The seed-vessel is open atone end, and the seeds which 

 lie loose at the bottom are thrown out by the swaying of the plant in 

 the wind or by the disturbance of a passing animal. A familiar 

 instance is afforded by the larkspur Delphinium, where the seeds collect 

 at the bottoms of the follicles, and are easily scattered by shaking or 

 striking the plant. Other instances are to be found in Hildebrand's 

 well-known book on the dispersal of seeds (i). In some of the Labiates 

 a condition of things biologically similar exists, but here the calyx 

 plays the part of the censer, and it is the nutlets, not the seeds, that 

 are shaken out. 



In some cases arrangements are found which seem to render the 

 dispersal more difficult, but it has been suggested that there is here a 

 contingent advantage to the plant, because the period during which 

 the seeds continue to be scattered is prolonged, besides which all the 

 seeds are not likely to be thrown out in one direction, which would be 

 the case if they fell out easily at the first waft of wind. The most 

 usual arrangement is that the capsule stands upright on its stalk with 

 a small opening at its upper end. Then, again, it often happens that 

 the seeds are not free, but adherent to the placenta, so that a some- 

 what violent shock is needed to displace them. In the case of the 

 Labiates it seems possible that the hairs which line the calyx in some 

 species play the same part. 



As a further refinement come those cases in which the capsule 

 does not remain permanently open. Steinbrinck (2), Kronfeld (4), 

 Beck (5), and others have described cases in which the mouth of the 

 capsule closes in wet weather and opens again when the air is dry. A 

 biologically parallel case has been noted by Rathay (3) in certain 

 Compositae where the involucre bends in and encloses the achenes in 

 moist air. A similar state of things is described lower down among 

 the Labiates. 



The special advantage gained by these adaptations is not obvious. 

 We have no evidence that the plant profits by the seeds or fruits 



1 Read at the Oxford meeting of the British Association, 1894. 



