STRUCTURE.] HOW DISTINGUISHED FROM ARILS. 37 



value in cases like that of Euonymus. From the preceding 

 facts I draw the following conclusions, which will place this 

 subject in a better light. 



The true aril, an accessory integument of the ovule, is 

 developed round the hilum as the proper integuments are, 

 and covers or would cover the exostome, if we suppose it 

 extended over the whole of the ovule. 



The false aril (arillode} of Euonymus, &c., an expansion 

 of the edges of the exostome, is often bent back around this 

 opening, but always leaves it exposed. 



We can distinguish, even in the seed, the nature of an 

 arilliform envelope, by the place of the micropyle which repre- 

 sents the exostome of the ovule. If the micropyle is hidden 

 by the envelope, or if it would be hidden, if the envelope were 

 extended further, we have a true aril. But if, on the con- 

 trary, the micropyle is not covered by the envelope and 

 cannot be, even if the latter is extended, then we have a false 

 aril like that of Euonymus. 



When an aril, true or false, forms around an anatropal 

 seed a bag open at its extremity, it can be easily distinguished 

 from a proper envelope, inasmuch as the latter is covered by 

 the raphe, and its opening, the micropyle, is in a direction 

 opposite to that of the arilliform sac. 



If the seed is orthotropal and we find a true aril, the 

 opening of the latter is turned to the same side as the micro- 

 pyle, i. e. towards the apex of the ovule ; in this case, of which 

 I know of but one example, (Cytinus hypocystis), the aril is 

 blended with the proper integuments of the seed. If, in an 

 orthotropal seed there were an arillode in the form of a sac, 

 it could not be confounded with either a true aril or with a 

 proper integument, because its opening would be next the 

 base of the seed on the side of the point of attachment. 



B. Of the True Aril True arils, though they resemble 

 each other in all essential points, are yet of extremely various 

 forms and sizes. We see them, even in allied species, take 

 every degree of extension from the annular rim, that hardly 

 surrounds the base of the seed, to the sac that entirely covers 

 it. These different states may be found in Dilleniads. 



