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 ^) and in Ricinus (see fig. 459 ^% where the seeds are thrust 

 through an opening of definite shape. In the Acanthaceae {Justida, 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 ^). 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: — 



Name of Plant. 



Cardamine impatie^is .. 



Viola canina , 



Dorycnium decumhens . . 

 Geranium columbinum 



Geranium palustre 



Lupimis digitatus 



Acanthus mollis 



Mura crepitans 



Bauhinia purpurea .... 



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 projection 

 of the most powerful contrivances for expulsion, viz. 15-0 metres, is inappreciably 

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

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

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

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

 mine impatiens, Dentaria, Impatiens, Lathrcea clandestina, Mercurialis perennis, 

 Orohus vernus, OxalisAcetosella, Viola canina, and V. sylvatica all inhabit retired and 

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

 sylvestris, climb over bushes and hedges on the borders of woods. Mention must 

 also be made of the fact that in many cases a second mode of dispersing fruits 

 and seeds acts conjointly with that of expulsion, as is indicated by the name of 

 Impatiens Nolitangere, i.e. "Touch me not". Those sling-fruits, for in.stance, 

 in which the high degree of tension is due to the swelling up and turgidity of 

 particular layers of cells, are so constructed that the slightest touch on the outside 

 causes a relaxation of the tension and the ejection of the seeds in the direction of 

 the object that has touched the fruit. The animals which frequent the shady woods 



