266 PHYSIOLOGICAL DEVELOPMENT. 



terizes the leaves for some time after they are unfolded. 

 Occasionally the traces of it are permanent; and, as in 

 the scarlet terminal leaves of Poinsettia pulcherrima, we see 

 that it may become, and continue, extremely conspicuous. 

 The question, then, now to be asked is has this colouring 

 by which the immature part of the phaenogamic axis is cha- 

 racterized, anything to do with the colouring of flowers? 

 Has this difference between undeveloped folia and folia that 

 are further developed, been increased by natural selection 

 where an advantage accrued from it, until it has ended in 

 the strong contrast we now see? I think we may not irra- 

 tionally infer that this has happened. 



Facts, very numerous and varied, united to warrant us in 

 concluding that gamogenesis commences where the forces 

 which conduce to growth are nearly equilibrated by the forces 

 which resist growth ( 78) ; and the induction that in plants, 

 fertilized germs are produced at places where there is an 

 approach towards this balance, we found to be in harmony 

 with the deduction that an advantage to the species must be 

 gained by sending off migrating progeny from points where 

 nutrition is failing. Other things equal, failure of nutrition 

 may be expected in parts which have the most remote or most 

 indirect access to the materials furnished by the roots 

 materials which have to be carried great distances by a very 

 imperfect apparatus. The ends of lateral axes are therefore 

 the probable points of fructification, in aggregates of the 

 third order that have taken to growing vertically. But if 

 these points at which nutrition is failing, are also the points 

 at which the colours inherited from lower types are likely to 

 recur in more marked degrees than elsewhere; then we may 

 infer that the organs of fructification will not unfrequently 

 co-exist with such colours at the ends of such axes. How 

 may the resulting contrast between the older fronds and the 

 fronds next the germ-producing organs be increased? If 

 uninterfered with it would be likely to diminish. These 

 traits inherited from remote ancestry might be expected 



