THE OUTER TISSUES OF PLANTS. 265 



so as to produce an under and an upper surface; and here 

 the two surfaces of the pseudo-leaf are slightly unlike in 

 contour if in nothing else. 



275. We now come to such physiological differentiations 

 among the outer tissues of plants, as are displayed in the 

 contrasts between foliar organs on the same axis, or on 

 different axes contrasts between the seed-leaves and the 

 leaves subsequently formed, between submerged and aerial 

 leaves in certain aquatic plants, between leaves and bracts, 

 and between bracts and sepals. To deal even briefly with 

 these implies information which even a professed botanist 

 would have to increase by special inquiries, before attempting 

 interpretations. Here it must suffice to say something 

 respecting those marked unlikenesses existing 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 folia of a 

 phsenogamic axis have sundry characters in common with such 

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

 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 de- 

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

 simpler Archegoniates. We also noted between them another 

 resemblance. It is said of the Jungermanniacea, 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- 



