most wanted. As the heart- wood forms, it is more 
and more separated from the living part, the al- 
burnum ; its functions become extinct, it dimi- 
nishes, dies, and at 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 ob- 
servations 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 sup- 
poses 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 , the great sources of the permanent 
beauty of vegetation, though infinitely diversified 
in their forms, are in all cases similar in interior 
organization, and perform the same functions. 
The alburnum spreads itself from the foot-stalks 
into the very extremity of the leaf; it retains its 
vascular system and its living powers ; and its pe- 
culiar tubes, particularly the tracheae, may be dis- 
tinctly seen in the leaf. m 
The green membranous substance may be con- 
sidered as an extension of the parenchyma, and 
the fine and thin covering as the epidermis. Thus 
* Fig. 1 1 . represents part of a leaf of a vine magnified and 
cut, so as to exhibit the trachese ; it is copied, as are also the 
preceding figures, from Grew’s Anatomy of Plants. 
