REPRODUCTION AND DISPERSAL 



919 



The dehiscence of fruits. — Fruits that open on maturity, thus per- 

 mitting the ready scattering of seeds, are known as dehiscent, while those 

 that do not open are called indehiscent. Dehiscent fruits are illustrated 

 hy capsules (figs. 1213, 1214) a,-ndpods (fig. 1212), while 6emM (fig. 1222), 

 stone fruits {drupes), and acorns (fig. 1223) represent indehiscent fruits. 

 Many indehiscent fruits are one-seeded, and may easily be mistaken for 

 seeds; among such are the small, dry fruits, known as achenes, especially 

 characteristic of the composites (figs. 1217, 1220), and also grains, nuts, 

 and acorns. In the umbellifers the fruits are known as schizocarps, the 

 one-seeded carpels splitting at maturity but not dehiscing (fig. 1221). 

 Although the habit seems relatively useless, dehiscence occurs in some 

 one-seeded fruits, as in the nutmeg. 



Usually the opening of dehiscent fruits is due to an unequal contrac- 

 tion of the pericarp tissues, resulting from desiccation, some cells or tis- 

 sues losing more water than do others; often the cells are arranged trans- 

 versely on the concave side, and longitudinally on the convex side. 

 Capsules usually open in such a way as to expose as many valves as there 

 are carpels, the splitting taking place along the separating walls or along 

 the middle line of the individual carpels. Characteristic pods are illus- 

 trated by the crucifers and legumes; among the latter the valves not only 

 are separated by longitudinal splitting, but there may be torsion within 

 the individual valves (fig. 12 12). Some fruits dehisce through pores, 

 as in the poppy; and in others the lines of dehiscence are transverse 

 rather than longitudinal, resulting in the detachment of the top like a 

 Hd, as in Portidaca and Plantago. At the time of dehiscence the seeds 

 readily become detached from the carpel wall and are exposed to dispers- 

 ing agencies; the scar left on the seed at the point of attachment is known 

 as the hUum. In the pines and in other conifers the cone scales commonly 

 separate from one another at maturity, exposing the winged seeds to the 

 wind; in some species the persistent cones may remain closed for many 

 years (as in Pinus Banksiana), the seeds thus retaining their viability 

 much longer than otherwise. In some of these trees, extreme desicca- 

 tion, such as is caused by forest fires, seems necessary to effect the open- 

 ing of the cones. In some indehiscent fruits there is an outer dehiscent 

 envelope, as in the involucre of the chestnut and the hickory nut and 

 in the aril of the bittersweet. 



The dispersal of fruits and seeds. — Dispersal by propulsion. — In 

 dehiscent fruits it is generally the seed, and in indehiscent fruits, the 

 fruit as a whole, that is scattered. In some cases the act of dehiscence 



