378 MORPHOLOGY OF THE ANGIOSPERMS 



fruit, may dehisce along ventral, dorsal, or both lines. Septicidal de- 

 hiscence consists, in part, of freeing of the individual carpels of a 

 syncarpous ovary by splitting along the lines of carpel union. Opening 

 of the carpels, so separated, is either along the ventral suture, as in 

 Tofieldia and other primitive liliacean genera, by breakdown of the 

 lateral walls in various degrees, or by a combination of these methods. 

 In free carpels, dehiscence may be through either dorsal or ventral 

 carpel margins, or through both — some Urticaceae, Rosaceae, Legumi- 

 nosae. The primitive method of dehiscence is doubtless loculicidal. 



Type of dehiscence is determined by anatomical structure, basic and 

 secondary, gross and minute, but form of the vascular skeleton is usu- 

 ally less important in dehiscence than histological structiue. Type, 

 amount, and distribution of fibrous, stony, and soft tissues determine 

 position of dehiscence that results from drying, the common condition 

 in nonfleshy fruit. In fleshy fruits that dehisce, abscission layers similar 

 to those in leaves and stems may be formed. Gross vascular structure 

 may bear little or no relation to lines of dehiscence. In circumscissile, 

 valvular, and poricidal dehiscence, the line of separation may cut across 

 vascular bundles. Histologically, dehiscence of fruits resembles abscis- 

 sion of leaves and stems but is structurally much more complex. Open- 

 ing is usually along definite lines, but some fleshy and semifleshy fruits 

 dehisce irregularly — Thalassia. On germination, indehiscent fruits com- 

 monly break open along structural lines, as a sort of delayed dehiscence, 

 but opening under pressure of the enlarging embryo may be irregular. 

 Some achenes and nuts have definite dehiscence — some Amaranthaceae, 

 Polygonaceae, Typhaceae; one horticultural variety of strawberry has 

 dehiscent achenes. 



Delayed dehiscence — "dormancy"^of fruits occurs in the fruits of 

 some xerophytic genera; they may remain indehiscent for several or 

 many years — those of Proteaceae and Myrtaceae, especially Hakea, 

 CalUstemon, Melaleuca, some species of Eucalyptus. The bases of the 

 inflorescence peduncle and the fruits themselves may be buried in the 

 bark and wood of the stems that bear them before the fruits open. (The 

 cones of some species of Pinus become similarly somewhat sunken in 

 branches before shedding.) Fruits of some species of Eucalyptus re- 

 quire over a year to mature and may not dehisce for many years, ap- 

 parently not until their water supply ceases and the twig bearing them 

 dies. In CalUstemon, the pericarp, with some chloroplasts, continues slow 

 growth for ten to eighteen years, dehiscing after three to twenty years. A 

 cork cambium builds up a firm, corky surface. The cause of the abundant 

 development, after forest fires, of seedlings of these xerophytes with 

 long-delayed fruit dehiscence seems to be the killing of the twigs that 

 bear them and the consequent shrinking and opening of the fruits. 



