xr.] NATURE OF THE EMBRYO 93 



equal in size, they lay the foundation of a 

 dicotyledonous embryo ; but if they be unequal 

 [i.e. by the arrest of further development], then 

 the larger one only becomes a cotyledon. 1 



It must not be overlooked that, while these 

 two terminal cells are regarded as the forerunners 

 of the two cotyledons, they have to supply the 

 axis and plumule as well between them ; so 

 that they really are of a "thalloid" character. 

 In dicotyledons this axis is continuous with the 

 root in a straight line ; but in monocotyledons 

 it is curved towards one side in consequence 

 of the growth of the single cotyledon, one 

 cotyledon being thus seen to be arrested from 

 the earliest stage. From numerous figures by 

 other observers, the inequality observed by Van 

 Tieghem does not appear to be always present, 

 but the energy of growth is what is arrested in 

 the one and not in the other cotyledon. 2 This 

 superior vigour soon makes the cotyledon to 

 grow apparently in the same line with the axis ; 

 but it is not really " terminal," the apex of the 

 plumule being the actual end of the axis. The 

 first leaf is usually opposite to, but not at the 

 same level or node with the cotyledon. This 

 gives rise to the "distichous" arrangement of 

 leaves seen almost universally in monocotyledons. 

 The laws of phyllotaxis, however, permit of three 



1 Ann. des Sci. Nat., 5 ser. torn. xiii. p. 24. 



2 " A Theoretical Origin of Endogens from Exogens, through Self- 

 Adaptation to an Aquatic Habit " (Journ. Lin. Soc., vol. xxix. p. 485). 



