It is evidently then only an organ of secondary 

 importance. In early shoots, in vigorous growth, it is 

 filled with moisture, and it is a reservoir, perhaps, of 

 fluid nourishment at the time it is most wanted. As 

 the heart- wood forms, it is more and more separated 

 from the living part, the alburnum ; its functions be- 

 come extinct, it diminishes, dies, and last disappears. 



The tendrils, the spines, and other similar parts 

 of plants are analogous in their organization to the 

 branches, and offer a similar cortical and alburnous 

 organization. It has been shown, by the late obser- 

 vations of Mr. Knight, that the directions of tendrils, 

 and the spiral form they assume, depend upon the 

 unequal action of light upon them, and a similar 

 reason has been assigned by M. Decandolle to account 

 for the turning of the parts of plants towards the sun ; 

 that ingenious physiologist supposes that the fibres 

 are shortened by the chemical agency of the solar 

 rays upon them, and that, consequently, the parts 

 will move towards the light. 



The leaves j the great sources of the permanent 

 beauty of vegetation, though infinitely diversified in 

 their forms, are in all cases similar in interior organi- 

 zation, and perform the same functions. 



The alburnum spreads itself from the foot-stalks 

 into the very extremity of the leaf ; it retains its vas- 

 cular system and its living powers ; and its peculiar 

 tubes, particularly the tracheae, may be distinctly seen 

 in the leaf,* 



* Fig 11. represents part of a leaf of a vine magnified and cut, so as to exhi- 

 bit the trachea ; it is copied, as are also the preceding figures, from Grew's Ana- 

 tomy of Plants. 



