36 
GARDENING AS A SCIENCE. 
in the middle, and the solid woody centre of the root proceeds in its growth 
in a corresponding ratio, lengthening as the stem lengthens, and increasing in 
diameter as the leaves unfold, and new woody matter is produced : the result of 
which is, that when the young exogen has arrived at the end of its first years 
growth, it has a root with a solid woody axis, and a stem with a hollow woody axis 
surrounding cellular tissue, the whole being covered in by a cellular integument.” 
The whole of the foregoing extract is consistently in accordance with the 
previous positions ; and, therefore, we only remark, by way of caution, that the 
tyro in physiology must not take it for granted, — although a woody stem be assumed 
as the type, that the theory does not equally apply to exogenous stems of all kinds, 
whether the plants be herbaceous or otherwise ; — for example, those with spindle 
{fusiform) roots, as carrot, beet, and the like, which afford, under the microscope, 
evident marks of a corresponding structure ; but of this our proofs will not be 
complete till we investigate the nature and offices of the foliage. Again : — 
“ 5. But as the woody cords are merely plunged into a cellular basis, the latter 
passes between them in a radiating manner, connecting the centre with the circum- 
ference by straight passages, often imperceptible to the eye, but always present.” 
Two^dra wings are then given, which convey an idea of the cross sections of 
the root and stem at the end of the first year ; they exhibit in the former the 
radiation of the fibrous and cellular matter from a solid centre , toward the outer 
covering integuments ; and in the latter a central cellular pith, (medulla) a series 
of alternating rays, and an exterior integument. 
“ 6. Here, then, we have the origin of Pith in the central cellular tissue of the 
stem, of wood in the woody axis, of hark in the cellular integuments, and of 
medullary processes in the radiated passages of cellular tissue connecting the centre 
with the circumference.” 
Our extracts are faithful to the minutest point of theory, though not precisely 
literal ; and we are not aware of any description better calculated to convey 
a correct idea of observable facts, always, however, with the admission that 
nature “ sports,” and is not tied down to preciseness of conformation. 
We make no allusions to the functions of the “ organism” in this place, nor 
can we, to do the subjects justice, enter into a description of the other classes of 
stems ; but it is essential to attend minutely to the first remark of No. 6 — 
“ Here, then, we have the origin of Pith,” because it involves two opposing 
theories. 
Many writers, Du Hamel among others, conceived that the medullary rays 
originated in the Pith. Dr. Keith repudiated the idea, and adduced the authority 
of the late Mr. Knight, which instructs us that in tracing the operation of budding 
he observed that the wood formed under the bark of the inserted bud, unites 
indeed confusedly with the stock, though still possessing the character and 
properties of the wood from which it was taken, and exhibiting divergent layers of 
new formation , which originate evidently in the hark , and terminate at the line of 
