142 ON SEEDLINGS 



obovate, emarginate, the two measuring 25 and 27*5 mm. in 

 length respectively, while cases occur in which they are rotund, 

 emarginate, broader and shorter, measuring 17*5 and 15 mm. 

 respectively, the latter being about 1 mm. wider than those 

 of the first mentioned. 



In several of the Sisymbrieee and Lepidineee the cotyledons 

 are minute ; this seems to be due partly to the small size 

 of the seeds, partly to a limited range of growth. The latter 

 case might arise from the embryo in the seed contain- 

 ing a relatively small amount of reserve-material, or more 

 probably from the plumule developing early and diverting 

 the energies of the plant away from the cotyledons. The 

 latter are shortly oblong, obtuse, with a long petiole in 

 Sisymbrium runcinatum (fig. 164), and S. Sophia, and linear- 

 spathulate with rather long petioles in Capsella Bursa- 

 pastoris (fig. 170) and Senebiera didyma. The cotyledons of 

 all the four species, but especially of Senebiera, recall those of 

 Papaver in their small size and narrowness. 



Linear cotyledons occur in Heliophila amplexicaulis 

 (fig. 167), H. crithmifolia, and probably in all the rest of the 

 genus. This type however is quite unusual in the Cruciferse. 

 The former of the two species has linear, obtuse, entire, 

 one-nerved cotyledons, tapering insensibly or gradually into 

 a short petiole and measuring from 20-25 mm. in length. 

 Those of H. crithmifolia are linear, acute, entire, subterete, 

 distinctly channelled all along the upper surface, sessile, 

 slightly narrowed to the base, thinly pilose, or pubescent, 

 20-38 mm. long, and becoming disarticulated from the stem 

 at an early age. 



It is probable that the seedlings of Palmstruckia, Cyclo- 

 ptychis, Carponema, and Chamira have also linear cotyledons. 

 Spirorhynchus has narrowly linear, incumbent cotyledons. 

 Those of Menonvillea trifida are incumbent, long, narrowly 

 linear, acute, sessile, and glabrous ; in the early stages they 

 are circinately coiled. 



In all these cases the narrowness of the cotyledons is due 

 to the shape of the seed, the position of the embryo with 

 relation to the broad and narrow planes of the seed, and the 

 peculiar folding of the embryo. 



