834 



THE DISPERSION OF SPECIES BY MEANS OF FRUITS AND SEEDS. 



In one class of expulsive fruits the high degree of tension which finally results 

 in the disruption and rolling up of particular tissues is caused by a swelling up of 

 the cell-membranes or by the turgidity of the cells. One of the most curious 

 instances is that of the Squirting Cucumber (Ecballium Elaterium), which is shown 

 in fig. 458 1. This plant belongs to the Cucurbitaceae and its fruit resembles a small 

 fleshy cucumber beset with bristles and borne by a hooked stalk. The end of the 

 stalk projects into the interior of the fruit like a stopper. When the seeds are quite 



Fig. 458.— Sling-fruits. 



' Ecballium Elaterium ; branch bearing flowers and fruits. 2 A fruit detaclied from its stalk and with its seeds squirting out. 

 3 Oxalis Acetosella; entire plant with one unripe fruit on a hooked stalk, and one ripe fruit on an erect stalk ejecting 

 its seeds; nat. size * Unripe fruit of Oxalis Acetosella; x6. * Ripe fruit of Oxalis Acetosella ejecting the seeds; x6. 



ripe the tissue surrounding them is transformed into a mucilaginous mass. Also 

 the tissue in the neighbourhood of the conical stopper just referred to breaks down 

 at the same time, and thus the connection between the stalk and the fruit is loosened. 

 In the wall of the fruit there is a layer of cells which is under great tension, and 

 endeavours to stretch itself out. As long as the fruit is unripe such expansion is 

 prevented by the tense tissue close to the stalk, but with the ripening of the fruit 

 this obstacle is removed. The fruit then severs itself from the conical end of the 

 stalk and at the same moment the expansion of the strained layer of tissue takes 

 place. The consequence is that the interior of the fruit is subjected to great 

 pressure, and the seeds, together with the surrounding mucilage, are squirted out 



