174 Mr. Knight on the Origin and Formation of Roots. 
cuttings abovementioned. The alburnum of the root is also 
similar to that of other parts of the tree, except that it is more 
porous, probably owing to the presence of abundant moisture 
during the period in which it is deposited.* And possibly the 
same cause may retain the wood of the root permanently in 
the state of alburnum ; for I have shewn, in a former memoir, 
that if the mould be taken away, so that the parts of the larger 
roots, which adjoin the trunk, be exposed to the air, such 
parts are subsequently found to contain much heart wood.* 
I would wish the preceding observations to be considered 
as extending to trees only, and exclusive of the palm tribe ; 
but I believe they are nevertheless generally applicable to 
perennial herbaceous plants, and that the buds and fibrous 
roots of these originate from substances which correspond 
with the alburnum and bark of trees. It is obvious, that the 
roots which bulbs emit in the spring, are generated by the 
sap which descends from the bulb, when that retains its na- 
tural position ; and such tuberous rooted plants as the potatoe 
offer rather a seeming than a real obstacle to the hypothesis 
I am endeavouring to establish. The buds of these are gene- 
rally formed beneath the soil ; but I have shewn, in a former 
memoir, that the buds on every part of the stem may be made 
to generate tubers, which are similar to those usually formed 
beneath the soil ; and I have subsequently seen, in many in- 
stances, such emitted by a re-produced bud, without the calix 
of a blossom, which had failed to produce fruit ; but I have 
never, under any circumstances, been able to obtain tubers 
from the fibrous roots of the plant. 
The tube therefore appears to differ little from a branch, 
* Bhil. Trans, for 1801. 
