THE SEED 373 



tracheidlike cells has been described as coating the embryo like a third 

 integument. This tissue perhaps represents a sheath of vascular strands 

 of the outer integument. 



The Aril. The fleshy coats and appendages of some seeds have various 

 origins. They may represent fleshy transformations of the outer cell 

 layers of the outer integument; proliferations of parts of this integu- 

 ment, of the chalaza, or of the distal part of the funicle. Those that 

 arise at the base of the ovule from annular primordia and develop 

 acropetally, sheathing the integuments more or less completely, are 

 called arils (see also under the Ovule). But the term aril is commonly 

 used loosely to cover any fleshy, external part of the seed. Definitions 

 usually restrict this term to sheathing coats developed from the base 

 of the ovule. Those developing from the micropylar rim of the outer 

 integument have been called false arils or arilloids. Some fleshy coats 

 apparently develop from both ends of the ovule. Development of an 

 aril may begin before fertilization but commonly begins later, while 

 the ovule is enlarging. The term aril has sometimes been used to cover 

 all modifications of the outer integument in the seed, even including 

 some that are not fleshy, such as wings and spines. Where fleshy tissues 

 are restricted to crests, as along the raphe, they are termed strophioles; 

 where restricted to the base or to the apex of the seed, they are termed 

 caruncles (Fig. 138A). These terms are confused in usage. Perhaps the 

 only important distinction among external fleshy parts of the seed is be- 

 tween those that represent transformed layers of the integuments, 

 chalaza, or funicle and those that are new parts of the ovule, secondary 

 proliferations of the original ovule body, such as the so-called third 

 and fourth integuments. (There is no convincing evidence that any 

 taxon has more than two integuments.) Fleshy layers of the outer in- 

 tegument are conspicuous in Lijcopersiciwi, Passiflora, Ptinica, Carica. 

 Seeds that mature free from the ovary have integuments differentiated 

 as in fruits; in CaulophijIIum, the seed resembles a fruit in its fleshy 

 outer and stony inner coats. Fleshy seed coats were long ago considered 

 proliferations of the ovary wall or the placenta; those of the tomato 

 have been called placental. Arils have been interpreted as third and 

 fourth integuments. In the large genus of the Proteaceae, Hihhertia, 

 arils, which are often called integuments, are merely elaborations of the 

 outer normal integument; the various species show all stages in the de- 

 velopment of an enclosing aril, from a fringed tip of the outer integu- 

 ment to a complete fleshy sheath. 



The possession of arillate seeds has been considered a primitive 

 angiosperm character, but this claim seems unlikely, because arils occur 

 in taxa scattered throughout angiosperms. Arils appear to be ecological 

 modifications related to dissemination. 



