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THE COURSE OF THE SAP IN PLANTS. 
163 
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THE COURSE OF THE SAP IN PLANTS. 
By J. TOWERS, Esq., C.M.H.S. 
I 
I 
I 
7110 bring to a close the question of the positive descent of the sap by gravitation or otherwise (a 
IX phenomenon which appears by the strictest analogy to have no real existence), the following pas¬ 
sage is extracted from the article by Dr. Henslow already alluded to :—“ That superabundant develop¬ 
ment of new matter which takes place round the edges of a wound, forming a swollen border, may 
probably arise from some additional stimulus produced by the morbid condition under which the tissue 
is placed. Thus the expansion of galls, and other excrescenses, is induced by punctures and injuries at 
particular spots, and exhibits the results of a morbid action to a greater extent than we see in the edges of 
such wounds as are inflicted by pruning.” Perhaps here, as in the case of ligatures also, where the 
upper edge of the bark becomes more enlarged than the lower edge, “ a greater energy may sometimes 
be exerted by the developing forces in a downward direction than in an upward, and yet this may be 
quite irrespective of the course of the sap.” 
Some years ago, a case in point occurred, where a ligature of thin iron bell-wire was passed round 
a lateral branch of a pear tree that projected too much over a walk ; this branch was drawn up, and 
kept in the required position by the wire, which was attached to another and stouter part of the tree. 
The ligature was forgotten, but a large crop of fruit, in a future season, requiring to be thinned-out, 
an attempt was made on the bough in question, which, however, snapt short off at that place, when it 
was seen that the wire had entirely prevented the enlargement of the wood, which was scarcely 
thicker than a quill, a burr of cortical matter having formed at the wound, which buried and entirely 
concealed the wire. 
The effects produced by the operation of “ budding ” will throw some light upon the courses of the 
sap. As a familiar example, we will appeal to the budding of the Rose. Without pretending to dis¬ 
cuss the “ vexed question”—whether the shield of bark, with its bud, should retain its strip of albur- 
nous wood, or be totally deprived of it, even to the slightest fibre, provided always that the root of 
the eye that is by nature attached to the alburnum, and through it to the medullary sheath, remain 
entire—sound theory demands :—First, that the bark of the stock should detach itself freely from the 
sap-wood beneath it, and show the exposed surface to be rich in that proper laborated fluid, called by 
some physiologists cambium—identical with that organizable matter, the vital envelope or indusium of 
the late Mr. Main, who thus described it in his Illustration of Vegetable Physiology: —“ This slender body 
of vitality, or vital envelope, is constitutionally compound, not simple as such a thin tissue may be 
supposed to be, containing the rudiments of both root and buds ; and, moreover, is the source of all ac¬ 
cretion, whether as to magnitude or number of the parts produced.” The second theoretical condition 
is like the first—it claims that the bark and young wood of the strip detached, should separate, or be 
capable, from existing moisture, of being separated with a facility equal to that which is found to 
exist in the stock wherein it is to be inserted. But this is not all; we have now to consider how, 
and by what instrumentality, two distinct bodies, both, however, imbued with a principle of life, shall 
unite and form an entire and perfect plant—perhaps ultimately an enormous tree from the progressive 
developments of a tiny eye or bud, not the size of a radish-seed at the time of its insertion ; but yet, in 
all those developments, retaining its distinctive integrity of character. 
The late Mr. Knight, while tracing the results of budding, observed that “ the wood formed under 
the bark of the inserted bud unites, confusedly indeed, with the stock, though still possessing the 
character and properties of the wood from which it was taken, and exhibiting layers of new formation, 
which originate evidently in the bark, and terminate at the line of union between the graft and the 
stock.” Again, Mr. Knight observed, “ that if a portion of a stem be stripped of bark, so as to leave the 
surface of a small portion of the sap-wood (alburnum) uncovered, the wound will heal, first by means 
of new bark issuing from the edges, and gradually narrowing the extent of the wound; and then by 
the production of new layers of wood formed under the bark as before. The new wood will not, 
indeed, unite with the portion of alburnum that had been exposed to the air; but it will exhibit, on a 
horizontal section, the same traces of divergent layers as before, extending from the bark in which they 
originate , to the lifeless surface of the old wood.” The phraseology of this last paragraph is Keith’s, 
in his commentary upon Knight’s experiments, which had induced that profound physiologist to sub¬ 
stitute the word “ con\nrgent” for that of divergent, medullary processes. 
When a bud is inserted, adroitly as to manipulation, in accordance with correct theory, the juicy 
surface of its bark, and that of the alburnous Avood of the stock, become attached together by the com- \ 
pressive force of the ligature, as does a boy’s leather sucker by the expulsion of the air betAveen it and 
a stone beneath it. But in the bud and its living bark there exists a A r italized fluid which, conjointly 
