93 



root. In all of the more highly developed plants the ascend- 

 ing axis soon separates into two formations, differing essen- 

 tially from each other, which, undergoing constant change, 

 represent in various forms the whole super-terrestrial life of 

 the plant. They are the axis or the stem and the leaf. The 

 stem is the persistentpart of all the super-terrestrial forma- 

 tions ; it is opposed to the leaf as the bearing part ; it forms 

 the centre, the axis ; the leaf on the contrary appears as a pe- 

 ripherically secreted part, which can be imagined to be sepa- 

 rated here and there from the stem. M. v. Martius thinks 

 that the leaf may also be considered as a secondary and re- 

 ramified axile formation, but that with respect to its inner 

 form it is asymmetrical. 



The axile formation increases in length and in breadth, 

 and after some time divides into branches, thus resembling 

 exactly the descending axis. But an essential distinction 

 between the growth of the stem and that of the root is 

 said to consist in the first being developed at fixed periods, 

 while the root is less subject to periods of terrestrial life, and 

 is therefore in a constant state of growth at all times of the 

 year. 



The leaf is developed by unfolding itself: it is not merely an 

 extension and elongation, but an increase in breadth ; and this 

 vital principle is the cause of its deviating from the perpendi- 

 cular position of the axile formation. The common leaves are 

 destined to increase and ameliorate the saps by the absorp- 

 tion and secretion of gaseous fluids : not so when the leaves 

 have changed their form, when they have become metamor- 

 phosed ; a phaenomenon which constantly accompanies the 

 production of the germ. A sexual function is the end of all 

 leaf-metamorphoses : the pistil-leaf on the female side, the 

 pollen-leaf or anthers on that of the male, wind up the beau- 

 tiful play of metamorphosis. These sexual leaves do not im- 

 mediately succeed the green ones, but a series of changes of 

 other leaves intervene, which, however, also belong to the ge- 

 nerative system. They differ from the foliage by their form 

 and colour, and may be termed coloured leaves. The trans- 

 formation of common leaves into coloured and into sexual 

 leaves is called the ascending metamorphosis; the retrograde 

 metamorphosis designates on the contrary the change of me- 



