I5IO A TEXTBOOK OF THEORETICAL BOTANY 



A number of species of Viola disperse their seeds by a squeezing opera- 

 tion, which is fundamentally similar to methods found in many other 

 genera (Fig. 1379). The ripe capsules are triangular in section and open into 

 three boat-shaped portions or carpels, whose walls are composed of suc- 

 cessive layers of thin-walled cells, long curved sclerenchyma and broad, 

 thickened cells. These layers dry unequally and cause the contraction 

 of the sides, which press upon the hard, polished seeds and eventually 

 force them off their attachments and eject them, much in the way that 

 a cherry stone can be shot from between the fingers. By this means V. 

 canina and V. riviniana can reach distances up to 15 ft., though the average 

 flight is about 10 ft. Each carpel segment of the capsule is emptied of seeds 

 in turn, the seed nearest the apex going first and the basal ones last. 



Claytonia (Portulacaceae) has also a squeezing mechanism. Here the 

 capsule also opens into three segments, exposing three seeds arranged 

 triangularly. The seeds are slightly tuberculate, which affords some resist- 

 ance to the pressure of the carpels as they shrink inwards on drying. When 

 this resistance is overcome the seeds fly off, sometimes in succession, some- 

 times all together. 



A combination of splitting and sudden twisting of the carpels or of their 

 woody endocarps is also responsible for seed dispersal in the families of 

 Acanthaceae and Euphorbiaceae, in which the seeds are usually large and 

 heavy. Further examples are Alstroemeria, Dictamnus, and Streptocarpus. 



The woody capsules in Acanthus are two-valved and contain 2 to 4 flat 

 seeds, each attached by a funicle which is curved and becomes woody and 

 is called a retinaculum. When the valves suddenly separate, the retinacula 

 spring up and straighten, shooting out the seeds to a distance of 20 to 25 ft. 



Fig. 1380. — Hevea brasiliensis. Rubber 

 tree. Carpel splitting and ejecting 

 the one large seed. 



The trimerous capsules of the Euphorbiaceae have an endocarp of 

 fibrous cells, which are straight when moist but curved when dry. The fruit 



