our English tongue in its pithy Saxon word, 'pith,' separating all our
ideas of vegetable structure clearly from animal; while the poor Latin and
French 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 quædam 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