THE OUTER TISSUES OF PLANTS. 249 



respecting those marked unlikenesses that exist between the 

 tissues of the more characteristic parts of flowers, and the 

 tissues of the homologous foliar organs. 



It was pointed out in § 196, that the terminal parts of a 

 phaenogamic axis have sundrj' characters in common with such 

 fronds as those out of which we concluded that the phajno- 

 gamic axis has arisen by integration — common characters of 

 a kind to be expected. In their simple cellular composition, 

 comparative want of chlorophyll, and deficiency of vascular 

 structures, the undeveloped ends of leaf- shoots and the 

 developed ends of flower-shoots, approach to the fronds of the 

 simpler Acrogens. We also noted between them another 

 resemblance. It is said of the Jungermanniacew, that 

 " though under certain circumstances of a pure green, they 

 are inclined to be shaded with red, purple, chocolate, or other 

 tints ; ■' and answering to this we have the facts that such 

 colours commonly occur in the terminal folia of a phseno- 

 gamic axis when arrest of its development leads to the 

 formation of a flower, and that very frequently they are 

 visible at the ends of leaf-axes. In the unfolding parts of 

 shoots, more or less of red, or copper- colour, or chocolate- 

 colour, may generally be seen: often indeed it charac- 

 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 Poinsetiia 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 difierence 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 been the case. 



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

 concluding that gamogenesis commences where the forces 



