SLING-FRUITS. 



839 



to cut edgewise through the air, and it is the invariable rule for seeds to be so 

 ejected as to encounter as slight a resistance from the air as possible whatever their 

 shape may be. Contrivances for determining the direction in which the expelled 

 body is to move are rare. A first indication of some such adaptation occurs in the 

 Wood-sorrel (see fig. 458 3 ) and in Ricinus (see fig. 459 10 ), where the seeds are thrust 

 through an opening of definite shape. In the Acanthacese (Justicia, Acanthus, &c.), 

 the path of projection is determined by the circumstance of the seeds resting before 

 their expulsion against rigid curved bars springing from the partition- wall which 

 runs through the fruit (see fig. 459 8 ). The act of expulsion is usually accompanied 

 by a characteristic noise like that of the bursting of a bladder, and the sound 

 amounts to a regular detonation in the case of the dehiscence of the fruits of Hura 

 crepitans. The range of projection is least when the seeds are small and light, and 

 greatest when they are large and heavy, as is shown by the following table: 



It will be noticed that as a means of distribution the agency of expulsive fruits 

 is confined to a very restricted range. As compared with the distances to which 

 seeds are conveyed by other means, such as the wind, the range of project 

 of the most powerful contrivances for expulsion, viz. 15'0 metres, is inapprecia 

 small This may account for the facts, firstly, that expulsive fruits are produce 

 by comparatively few plants; and secondly, that such plants as do possess 

 for the most part denizens of localities that are sheltered from the wind, v 

 therefore the conditions are not favourable to dispersion by that agency. 

 mine impatiens, Dentaria, Impatiens, Lathrva clandestine Mercurialis perenn 

 Orobus vernus, OxalisAcetosella, Viola canina, and V. sylvatwa all inhabit retiree 

 shaded woodlands, whilst others, as, for instance, Geranium palustre and 



