on the Motion of the Sap in Trees . 187 
that which the vessels of the bark are immediately prepared to 
carry away. Much new wood will in consequence be generated, 
and increased vigour given; and, the same causes operating 
through successive seasons, will give the ascendancy we gene- 
rally observe in the perpendicular branch. 
In the preceding experiments, none of the layers, or cuttings, 
exceeded a few inches in length; and, to the summit of these 
the sap appeared to rise, through the inverted tubes of the wood, 
nearly as well as in those which retained their natural position. 
But some former experiments had induced me to suspect, that 
this would not be the case in longer cuttings ; I therefore planted, 
in the autumn of 1802, twelve cuttings of the sallow, ( Salix 
caprea,) inverting one half of them. The whole readily emitted 
roots, and grew with luxuriance; but their modes of growth 
were extremely different. In the cuttings which stood in their 
natural position, vegetation proceeded with most vigour at the 
points most elevated ; but, in the inverted cuttings, it grew more 
and more languid as it became distant from the ground, and 
nearly ceased, towards the conclusion of the summer, at the 
height of four feet. The new wood also, which was generated 
by these inverted cuttings, accumulated above the bases of the 
annual shoots, as in the preceding instances. 
These facts appear to prove, that the vessels of plants are not 
equally well calculated to carry their contents in opposite di- 
rections ; and, I think, afford some grounds to suspect that the 
vessels of the bark, like those which constitute the venous 
system of animals, (to which they are in many respects analo- 
gous,) may be provided with valves, whose extreme minuteness 
has concealed them from observation. 
The experiments, and still more, the Plates, of Hales, have 
B b 2 
