222 



PRINCIPLES OF PLANT-TERATOLOGY. 



case of the elm indications thereof were observed, one 

 of the two leaves resulting from the fork being slightly 

 displaced. The meaning of this will appear later. 



There must here be introduced the interesting cases 

 in which a single terminal leaf is replaced by a pair of 



Fig. 59. — TJlmus glabra. Diagrammatic representation of mode of 

 origin of opposite-decussate phyllotaxis from the distichous 

 arrangement. 



opposite ones, In the umbrella-pine (Sciadopitys ver- 

 ticiUata), on proliferation of the short-shoot bearing 

 the single "double needle," this organ splits into a pair 

 seated on opposite sides of the shoot, clearly a rever- 

 sion to the primitive condition. Now in Finns mono- 

 phylla, which normally bears a single terminal needle, 

 this is not constituted by the fusion of two, as in 

 Sciadopitys, but one of the original pair has become 

 suppressed, thus allowing the other to assume sole occu- 



