THE FRUIT 379 



Most indehiscent fruits are monocarpellary, but there are many ex- 

 ceptions, such as the fruits of the Fagaceae, Juglandaceae, Gramineae, 

 Betulaceae, many Palmae. 



Pericarp 



Where the matured ovary wall, the pericarp, is differentiated into 

 histologically distinct layers, these are distinguished as exocarp, meso- 

 carp, and endocarp — outer, middle, and inner layers, respectively — or 

 into exocarp and endocarp only. Distinction of these layers is generally 

 sharper in fleshy than in dry fruits. 



In the maturing of many fleshy fruits, as in Rubus and Fragaria, the 

 rapid enlargement in ripening is the result of cell enlargement, not of 

 new-cell formation. Apparently, in many kinds of fruits, cell multiplica- 

 tion in the pericarp ceases early, even at, or just after, flowering, as in 

 Rubus and Ananas. 



Stomata are frequent and lenticels occasional on fruits, especially on 

 the fleshy types, appearing at different times in different taxa as the 

 fruit enlarges, sometimes even in late stages of enlargement. They may 

 form on only the outer surface of the pericarp; this is the common con- 

 dition. But they may form on both outer and inner surfaces, as in cap- 

 sules — Papaver, Datura, Chelklonium, Hyoscyamus; they are rare on 

 the inner surface only — Capsicum. 



The Integuments in Fruits 



In indehiscent fruits and many dehiscent fruits, the inner integument 

 is commonly much reduced or lost, the outer remaining thin, or much 

 thickened and stony. Rarely, the inner alone survives; in the bamboos, 

 the outer integument is absorbed immediately following fertilization, 

 and the inner is reduced to a single layer of cells. Occasionally, both 

 persist, as in the palms, where the outer contributes to the formation 

 of the stony shell. In some achenes, especially in the caryopses of 

 grasses, the integuments are represented merely by vestiges; in the 

 millet, Panicum milliaceum, by the epidermis of tlie nucellus; in Zea, by 

 a noncellular suberized layer, a remnant of the nucellar epidermis. In 

 seeds maturing outside the carpel, as in Caulophyllum, the integuments 

 become massive and pericarplike, with fleshy outside and stony inside 

 layers. 



Evolutionary Relationships of Fruit Types 



The follicle is clearly the primitive fruit type, derived directly from 

 the primitive carpel. Modification of the free carpel produces many 

 types. Reduction of ovule number to one and loss of dehiscence pro- 

 duces the achene and other types. But the descriptive term achene is 



