rench must use the entirely inaccurate words 'medulla' and 'moelle'; all,
however, concurring in their recognition of a vital power of some essential
kind in this white cord of cells: "Medulla, sive illa vitalis anima est,
ante se tendit, longitudinem impellens." (Pliny, 'Of the Vine,' liber X.,
cap. xxi.) 'Vitalis anima'--yes--_that_ I accept; but 'longitudinem
impellens,' I pause at; being not at all clear, yet, myself, about any
impulsive power in the pith.[40]

14. However, I take up first, and with best hope, Dr. Asa Gray, who tells
me (Art. 211) that pith consists of parenchyma, 'which is at first gorged
with sap,' but that many stems expand so rapidly that their pith is torn
into a mere lining or into horizontal plates; and that as the stem grows
older, the pith becomes dry and light, and is 'then of no farther use to
the plant.' But of what use it ever was, we are not informed; and the
Doctor makes us his bow, so far as the professed article on pith goes; but,
farther on, I find in his account of 'Sap-wood,' (Art. 224.) that in the
germinating plantlet, the sap 'ascends first through the parenchyma,
especially through its central portion or pith.' Whereby we are led back to
our old question, what sap is, and where it comes from, with the now
superadded question, whether the young pith is a mere succulent sponge, or
an active power, and constructive mechanism, nourished by the abundant sap:
as Columella has it,--

"Naturali enim spiritu omne alimentum virentis quasi quaedam anima, per
_medullam_ trunci veluti per siphonem, trahitur in summum."[41]

As none of these authors make any mention of a _communication_ between the
cells of the pith, I conclude that the sap they are filled with is taken up
by them, and used to construct their own thickening tissue.

15. Next, I take Balfour's 'Structural Botany,' and by his index, under the
word 'Pith,' am referred to his articles 8, 72, and 75. In article 8,
neither the word pith, nor any expression alluding to it, occurs.

In article 72, the stem of an outlaid tree is defined as consisting of
'pith, fibro-vascular and [42] woody tissue, medullary rays, bark, and
epidermis.'

A more detailed statement follows, illustrated by a figure surrounded by
twenty-three letters--namely, two _b_ s, three _c_ s, four _e_ s, three _f_
s, one _l_, four _m_ s, three _p_ s, one _r_, and two _v_ s.

Eighteen or twenty minute sputters of dots may, with a good lens, be
discerned to pr