DIFFERENCES IN ALLIED SPECIES 



25 



In many cases, seeds of the same shape produce cotyledons 

 of very different form. 



Compare together, for instance, Euellia (fig. 55) and 

 Cerastium (fig. 23 seedling, and fig. 56 seed). Both have 



A 



SAIL 



FIG. 55. Ruellia longifolia. A, longitudinal section 

 of seed, x 10. B, transverse section of seed, x 10 : 

 FH, fringe of hairs ; LAu, larger, and SAu, smaller 

 auricle. 



compressed, nearly orbicular seeds, but in Euellia the coty- 

 ledons are broad, in the Chickweed they are narrow. If, 

 however, we make sections of the seeds, the cause of this 

 difference becomes obvious, because in one (fig. 55) the coty- 

 ledons lie parallel, in the other (fig. 56) transverse, to the seed. 



FIG. 50. Cerastium arvense, x 15. 



The form of the cotyledons often differs greatly even in 

 the same family. 



The Caryophyllaceas, for instance, afford us an interesting 

 illustration. The cotyledons in this family are placed with 

 their backs to the placenta, and in most species are narrow, 



