402 KEY. GH50BOE IIEXSLOW ON A THEORETICAL 



given by Irmisch, Ilegelmaier, Hanstein, and others, it does not 

 seem that this idea of equality, or the reverse, in the two cells 

 can be sustained. But that the energy of growth is arrested in 

 one and not in the other, appears to be the real interpretation of 

 a monpcotyledonous embryo. Hence one only elongates, its 

 superior vigour carrying it on in a straight line with the sus- 

 pensor, finally making the cotyledon terminal ; comparably, 

 perhaps, with a vigorous lateral shoot, as of vine or horsechest- 

 nut, w r hich finally assumes an axial direction. 



M. Ph. van Tieghem notices that the angular divergence between 

 the cotyledon and the next new organ is 180° (generally in 

 endogens) ; or, if lateral, 90° (as in exogens). When this latter 

 case occurs in endogens, it implies that the one cotyledon is due 

 to an arrested condition of the other of two opposite cotyledons ; 

 so that the first leaf belongs to a second pair decussating in 

 position with the cotyledons. If the first leaf be at 180°, then 

 it would appear to have usurped the position of the cotyledon 

 that is lost ; but it is not strictly on the same level as the 

 cotyledon, in as much as the first leaf is completely included 

 within it, as may be seen in Asparagus, figured by Irmisch *. 

 The cotyledon corresponds to one of the cords of the root. It 

 receives from the tigellum a single vascular cord which trifur- 

 cates on emerging. The second leaf receives also a three-forked 

 cord, and is found to be exactly opposite the cotyledon. The 

 third and fourth leaves are also opposite each other. The left one 

 of the two accompanying diagrams shows their positions; while 

 the arrows indicate the course which the spiral will take when 

 the arrangement of the leaf-scales becomes resolved into the 

 f system, as represented by the right-hand diagram. This is, of 

 course, particularly characteristic of phyllotaxis in exogens. 



2 



5 - 2 



5 



:} 4 



3 4 



6 



Cot. 



1 



* « 



Beitriige zur vergleichenden Morphologie der Pflanzen/ 18f>4, Taf. vii. 



i^r ^j ^ — - — — w - w 



figs. 4(3, 47. R. Brown observed that the first leaf of the plumule in Aroidea 

 is situate opposite to the cotyledon (Prod. PL Nov. Holl. p. 334). 



