1819.} Direction of the Radicle and Germen. 261 
zontal and revolving wheel? I may also add, that the lateral and 
horizontal shoots, issuing whether from the radicles of herbs, or 
even of trees and other woody plants, as well as the extreme 
fibres of the grand divisions of the root itself, are all as perfectly 
succulent as the most tender radicle, and as completely within 
the influence of gravitation. Indeed the germinating radicle is 
not always either very soft or very succulent. The radicle of 
the bean is a firm and compact substance, even at the time that 
it may be bending downwards. Let any one make the proper 
observations by watching the germination of a bean; or by tak- 
ing up the extreme fibres of the root of a tree, the elm tree, for 
example, at the distance of 10 or 12 feet from the trunk, and 
he will find that what I have now asserted, whether with regard 
to the radicle or to the lateral fibre, is the fact. 
Gravitation, therefore, if it were the sole, or even the main 
cause giving direction to roots, ought still to operate on the 
lateral fibres with its full effect. The resistance of the subsoil 
does not, in fact, present any very great difficulty to downward 
growth ; for it is penetrated by many roots that seem to be much 
less fitted for the operation than others that never approach it. 
Why does the root of Triticum repens creep along horizontally at 
the depth of two or three inches below the surface, though it is, 
perhaps, the best fitted of all roots for perpendicular descent, by 
its being furnished with a fine and stiff point that will often 
penetrate in a horizontal direction, through substances that are 
much more firm and compact than the soil in which it grows. 
Thus it has been known to make its way even through a potato, 
or Jerusalem artichoke, though it will not descend to any consi- 
derable depth; and yet if you sow carrots or parsnips in the 
same soil, their roots will descend to the depth of a foot or more. 
It is evidently the effect of an election in the plant. — 
With regard to the tap root of the oak, of whieh every body 
talks, I can say nothing from my own observation. Du Hamel 
asserts its existence,* and Mr. Knight denies it ; and from the 
number of trees which Mr. Knight examined, he certainly has a 
right to speak with some confidence ; though woodmen who 
have grubbed up many oak trees, uniformly affirm that they are 
often furnished with a tap root, extending, in most eases, to the 
depth of three or four feet, and thick in proportion to the trunk. 
But, however it may be with the oak tree, there are undoubtedly 
many plants of which the first and perpendicular radicle or root 
still continues to grow, and to be of the utmost importanee to 
_the individual, as is evident from the examples of the roots of the 
cartot, the parsnip, and the radish, of which the matured radicle 
constitutes the main bulk. 
In my paper, upon the development of the seminal germ, pub 
lished in the Transactions of the Linnean Society,} 1 stated, as 
» * Phys, des Arb. liv. i, chap. v, + Vol, XI. Part IT, 
