68 ON SEEDLINGS 



light. The embryo is straight ; the cotyledons linear, obtuse, 

 entire, closely applied face to face, with their edges to the 

 placenta. 



In P. arenaria and P. major the cotyledons are also placed 

 with their edges to the placenta. 



I was for some time much puzzled as to why the cotyledons 

 in P. media should be placed differently from those of the 

 other species examined ; though the reason seems in reality 

 very simple. At first I thought it might have reference to 

 the mode in which the embryo emerges from the seed ; but 

 this does not seem to have any bearing on it. In P. lanceo- 

 lata, however, and its allies the cotyledons are narrow and 

 thick ; and the seed being somewhat compressed, it will be 

 seen from fig. 113 that if the embryo had been placed with 

 the faces of the cotyledons to the placenta, it would not have 

 had room to develop. 



On the other hand, in P. media (fig. 112) the reverse is the 

 case : the cotyledons are thin and comparatively wide ; their 

 width, in fact, is greater than their thickness. It follows that, 

 if they had been arranged as in the other species, they would 

 not have had room to develop. The difference of position is 

 therefore explained by the fact that in P. media the width 

 of the cotyledons is greater than the thickness; while in 

 P. lanceolata &c., on the contrary, the thickness of the two 

 cotyledons, taken together, is greater than their breadth. 



The normal arrangement of an embryo in the seed is to 

 have the faces of the cotyledons turned to the placenta. 

 There are, however, not a few cases in which, as in these 

 species of Plantago, the cotyledons have their edges to the 

 placenta. When this is the case, it may be suggested as 

 possible that the position is due to the fact of the seeds being 

 more or less, in some cases very much, flattened ; and that 

 the embryo is twisted round at right angles to its normal 

 position, so that the cotyledons may lie in the broad way of 

 the seed, as in Ailanthus, Euonymus, Passiflora, Linum, 

 Fraxinus, Diospyros, Heliotropium, and many Crucifers, Legu- 

 minosse, and Kosacese. 



On the other hand, in the case of Claytonia (fig. 114) this 

 explanation will not apply. There would appear no reason, so 



