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ON THE CULTURE OF ASPARAGUS. 179 
from the leaves backward through the hark, he the currents either upward, downward, or in a 
horizontal direction. 
The upward current of the sap passes through all the woody mass of young trees, hut as their age 
increases the old wood becomes solidified, and loses the power of conveying fluids, so that the current 
passes through the outer and younger layers. The more solid the character of the wood of a tree the sooner 
it loses its active functions, and thus an Oak tree will soon dry up and die if a ring of its young wood 
be removed ; while in a tree with soft wood, like the Birch, for example, the inner portions of the wood 
convey sap even in very thick stems. The old wood which has become choked by the solidification of 
its cells, is known by the name of the heart-wood, or duramen, in contradistinction to the alburnum, or 
sap-wood. 
Such being the general facts of the process, we now have to examine into its details, namely, as to 
which of the elementary organs the sap rises through, and by what force it is caused to ascend ; and 
here we come to an inquiry full of obscurities, and rendered still more difficult by the numerous specu- 
lations that have arisen from the labours of those who have investigated it. Two most opposite 
opinions are mentioned in reference to the first of the above questions. AVhile some authors contend 
that the sap rises in the vessels or duets, others assert that these transmit only air, and that the sap 
flows through the cellular tissue. The strongest argument for the view that the vessels cany sap is 
furnished by the fact observed by several experimenters, that when plants have been made to absorb 
prussiatc of potash, sulphate of iron has subsequently shown the presence of this in the vessels ; but 
recent researches in the same direction, and in which the same experiments have been made, have 
yielded exactly opposite results, — without, however, furnishing any satisfactory clue to the contradic- 
tion ; it is possible that the solutions may have passed into the vessels, in the first instances, through 
some accidental injury to the plant. The opinion that the vessels carry air is founded upon micro- 
scopic examination, which alwa}'s shows air to be contained in them, excepting only under peculiar 
and temporary circumstances. In the winter a portion of the cells of the wood arc filled with sap, and 
the vessels contain air. As the season advances the sap becomes more abundant in the cells, and 
makes its way also into the vessels, so that if the wood is wounded in the spring, the sap flows freely 
out from the cut ends of the vessels, while it is not effused if it be contained in the cells alone ; after a 
time, when the unfolding of the leaves and the consequent great evaporation have removed much of 
the sap from the wood, the vessels are again found full of air. 
It would appear, from the observations of certain travellers, that this fulness of sap is constant in 
some climbing tropical plants, the vessels being always filled with it. When the sap is contained in 
the vessels, it appears to he subjected to a more or less considerable pressure, being forced out with a 
certain force when the wood is wounded, as was shown by the experiments of Hales upon the Vine ; 
yet, that this is not always the case would seem to be proved by the experiments of Gaudichaud and 
Poitcau on the tropical climbers before mentioned ; in them the sap did not flow from cither the upper 
or lower portion when a stem was simply cut across, but when a piece was taken completely out, by 
two cuts, the sap ran from whichever end was held downwards. 
With these exceptions, however, the vessels may be safely held to contain air. In the Vine and 
other wood}' plants during the bleeding season the cells are filled with sap, and the vessels do not 
receive it until subsequently ; after the unfolding of the leaves, when the evaporation has become 
active, the vessels again become emptied of sap. It may he concluded, therefore, that the cellular 
system of the wood is the tissue primarily and especially devoted to the function of conveying the sap, 
and that the vessels and ducts take part in it only under certain circumstances, temporarily in most 
plants, when the stem is overfilled with sap ; perhaps throughout the whole period of active vegetation 
in a certain number of plants possessing extraordinary abundance of sap. 
ON THE CULTURE OF ASrAltAGUS. 
I'.v jfn. J. TOWERS, Member op tin: Royal Aqbiouitobal and Eobtict ltobai Sooibtibs, >v<. 
Jfl QUESTION of considerable interest presents itself al this time, in consequence of communi- 
iC\ cations that have been published, and opinions hazarded, respecting the proper treatment of 
Asparagus beds and rows during the season of cutting for the use of the table : thai is. under ordinary 
circumstances, from about the first week of Maj to the -1st or 24th of June. Much difference of 
opinion prevails; and having myself been a propagator and grower of this plan) for a long period, in 
three remote counties, and under varieties of soil, aspect, and temperament — having, moreover, raised 
the plants from seed, experimented to a considerable extent, and attended to plots raised at periods 
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