SLING-FEUITS. 



835 



with considerable force through the hole which was previously closed by the end 

 of the stalk (see fig. 458 ^). 



The Dorsteniaceae behave in a manner no less remarkable. As in the case of 

 Figs so also in these plants, numbers of small flowers are seated upon an enlarged 

 receptacle, which remains fleshy and succulent after the small one-seeded fruits have 

 developed from the flowers. The lower portion of each fruit has thick walls, and is 

 embedded in the receptacle like a hair-follicle in the human skin, whilst the delicate- 



Fig. 469. 

 1 Orobus vernus. 2 and 8 Geranium pahtstre. * Viola elatior. 

 7 and 8 Acanthus mollis. 9 and 10 Ricinus communis. 



Sling-fruits. 



^ Cardamine impatiens. 



s Impatiens Nolitangere. 



coated portion projects above the receptacle in the form of a papilla. When the 

 seed is quite ripe the turgidity of the outer cellular layer of the thick wall of the 

 fruit increases, the thin-walled top is torn, the thick walls suddenly close, and the 

 seed hitherto enveloped by them is violently ejected. 



A special case of the expulsion of seeds as from a sling is also found in Oxali- 

 daceae, of which the common Wood-sorrel (Oxalis Acetosella, see figs. 458 ^- *■ ^) may 

 be taken as an example. In this case it is the seed -coat that possesses a special 

 tumescent tissue adapted to the expulsion of the seeds. One of the deeper layers 

 of the seed-coat is composed of tense cells and is itself in a highly strained condition, 



