This is not all that is odd about Matthew's re-quote. He did not only revert to Loudon's original usage at times, suggesting that he knew the original, but Loudon's work is also sprinkled with insights about natural selection and the effects of culture on domesticated varieties that seem highly relevant to the idea of natural selection (see here).
Now we come to what Steuart (1828) had to offer and that Matthew (1831) did not cite, because he did not simply re-quote the whole Loudon quote in Steuart, but spliced it.
Notice, first, that the passage in which Steuart (1828) quotes Loudon (1806) is an endnote. That is, on page 118f Steuart (1828) criticises what Mr Pontey (1806) has written on pruning in The Forest Pruner; or Timber Owner's Assistant. This criticism contains an asterisk referring to a footnote at the bottom of the page, which in turn refers to a Note III at the end of the book. Here is that passage from the main text of Steuart (1828, my highlight):
>>In this view, it will be perceived, that judicious Pruning is a work of far greater nicety and difficulty, than is generally believed, and that it should not be permitted, unless under the superintendence of some scientific person. It is true, it has been shown (and, I think, satisfactorily, by the ingenious Mr. Pontey), that severe pruning must augment the actual weight of the stem, and, as he specifically argues, the value of the tree. But great doubts may be entertained, whether, in some cases, this writer, meritorious as he is, may not have proceeded on an erroneous principle in his theory; and that his practice in pruning has been carried [page-break, 118/119] to a hight sanctioned by neither science nor experience.*
Branches, besides giving to Trees both beauty and nourishment, serve to balance them properly; and by throwing themselves out on every side, aid the Tree in withstanding the wind, in whichever way it may blow. Most Trees, if not prevented by adverse circumstances, have, at first, a leading shoot, which tends perpendicularly upwards, and is invested with a preeminence over the other branches. Having reached the height, which the soil and situation admit, the central shoot loses its preeminence. The sap, required to give it superior vigour, seems then to fail, and it gradually disappears among the other shoots. Meanwhile, the plastic powers of the Trees soon multiply the branches of the top, which gradually obtain a rounded form, and becomes what the nurserymen call "clump-headed. [...]
* Note III.<< (Steuart 1828, p. 118f)
The term plastic powers is highlighted, because certain plagiarism-theorists think that Darwin stole his use of the term plastic in relation to the traits of organisms from Matthew (1831). Steuart (1828, p. 119, see also 54, 85, 192, 198) proves that the term was probably as widespread among scholars as selection.
The Note III at the end of the book begins at page 396 and starts with enumerating the different effects of pruning and specifying what is wrong about Pontey's doctrine of pruning. In particular, he criticised that Pontey erred in believing that the sap flows only oneway from the roots up the tree, and that every side-branches cut away means more sap for the stem. Steuart (1828, p. 398) packed his devastating criticism into a rhetoric question and wonders why Loudon (1806) seems to have been the only one so far to criticise Pontey's ill advice on pruning, which feeds into a first quote from the "Encyclop. of Gardening" (which can be found at page 1104 in the first edition of 1822):
>>If such be the principles of science, on which this systems of Pruning is founded, there is little wonder, that it should prove erroneous, when applied to practice. What should we think, in the present day, of a scientific Agriculturist, who was unacquainted with the Chemical affinities? or of an astronomer, who assumed as the basis of a new system, that the Sun and Planets moved round the Earth? Yet it is singular, that the ingenious author of the Encyclopedia [sic, the title spells Encyclopaedia] of Gardening (himself a skilful Phytologist), is almost the only writer of note, who has ventured to cast doubt on this rash system of Pruning; or to observe the vast difficulty and delicacy, that attended so scientific an operation.
"The great importance (says he) of the Leaves of Trees must never be lost sight of. In attending to these instructions, their use is not, as Pontey asserts, "to attract the sap," but to elaborate it, when propelled to them, and thus form the extract or food taken in by the plant, into a fluid analogous to blood, and which is returned, so formed by the Leaves, into the inner bark and soft wood. It must be a very nice point, therefore, to determine the quantity of branches or leaves, that should be left on each Tree; and, if no more are left than what are just necessary then, in the case of accidents to them from insects, the progress of the Tree will be doubly retarded.<< (Steuart 1828, p. 398)
>>Perhaps there is no other author of the present time, who has written more judiciously on the effects produced on Wood by means of Culture; of which Pruning necessarily forms an important part, that the ingenious author of the Encyclopedia [sic] of Gardening: And I feel the more particular satisfaction in appealing to min, in this place, as I have above had occasion to differ from him on another point respecting wood. In a meritorious Treatise, which came out in 1806, I mean "On the Forming and Improving of Country Residences," and which I have already noticed (at p. 360) in the terms it deserves, he has an interesting article "on the Effects of Culture of Trees, in regard to characteristic beauty, and Timber produce."
"It is remarkable," he observes, "that this subject has never specifically engaged the attention of those, who have written on Planting. The Effects of Culture on other vegetables is so great, as always to change their appearance, and often, in a considerable degree, to alter their nature. The common Culinary vegetables, and cultivated grasses assume so different an appearance in our fields and gardens, from what they do in a state of wild nature, that evena [sic] botanist might easily be deceived, in regard to the species. The same general laws operate upon the whole kingdom of vegetables; and thence it is plain, that the effects of culture on Trees, though different in degree, must be analogous in their nature. ** [These asterisks probably indicate an omission of a long sentence on Steuart's part.]
"It may be proper to observe, that I, by culture, do not mean merely the operations on the soil, or even on the form of the form of the Tree, but every thing that tends to remove it from its natural state, in order to accelerate vegetation. I consider also, that a Tree is in a natural state, whenever it has sprung up fortuitously, and propagates itself without aid from man, whether it is in crowded forests, woody wastes, or in scattered groups on hills or commons. Now, it is known to everyone, in the least conversant with vegetable economy, that, in all herbaceous vegetables, and even in shrubs of considerable size, the effect of removal to an improved soil, climate, and situation, is to expand the parts of the whole vegetable; that the effect of removing, or cutting part of the vegetable above ground, is to expand those parts which remain; that the effect of removing any of the parts under ground, or of removing the whole vegetable into a colder climate, and a less congenial soil and situation, is to contract or consolidate the whole. This, were it necessary, could be illustrated in a thousand instances, from the commonest vegetables: But I only notice further at present, that this takes place more or less, in degree corresponding with the rapidity of the growth of the vegetable and its duration."
After a good deal of illustration and discussion, which is all admirable, and highly deserving of the phytologist's and planter's notice, he concludes as follows.<<Thereafter, Steuart (1828, p. 400-403) continues to quote Loudon (1806, p. 502-505) on the effects of pruning on trees, which Matthew (1831, pp. 295-298) chose to re-quote and to criticise (Matthew, 1831, pp. 298-308).
Whatever Loudon (1806, p. 502-505) has written in this passage must be seen in the context of his earlier criticism against Pontey's doctrine of simply pruning up a tree leaving the stem without any side-branches (only top branches). It must also be seen in the context of what Loudon has said on the effects of culture on plants.
One important point, here, is that Matthew cannot have been ignorant of this context, because Steuart (1828) fully transported it in the pages immediately preceding the ones that Matthew chose to criticise. Nevertheless, Matthew manages to produce the impression that Loudon was as mad a pruner as Pontey (for the sake of contrast, Matthew gets a different font).
"We differ from the author of the Encyclopaedia of Gardening here, even in limine, in his assumption, that pruning is of a corresponding nature with culture, in increasing the annual circles of the wood. Culture, if judiciously executed, increases these annual circles; but common pruning up (which, from the general bearing of the language, we suppose is meant), nine times out of ten diminishes them" (Matthew (1831, p. 299)Compare this with what Steuart (1828, p. 118f) has said on his own behalf against severe pruning and what he has quoted Loudon (Steuart 1828, p. 398) to have said: that the importance of leaves for the trees must never be lost sight of, that leaves produce the food for trees, and that pruning must therefore be a difficult operation that should not be overdone. In particular, if a tree is pruned up to leave the minimum of what he can tolerate, an accidental further loss, for examples through insects, will "doubly retard" the tree (see second quote given above).
Against this background, Matthew's suggestion that Loudon advocated common pruning up, leaving no side-branches on the stem but only top branches, that would injure the tree, is highly dubious.
Next, Matthew manages to produce the impression that Loudon thought that trees would grow best in their natural situation, when in fact Loudon said the main effect of culture was to enlarge all parts of the cultivated plant, no matter whether it was a tree, shrub, grass, or other vegetable (see also here). [The idea that trees grow best in their natural locations was, however, a tenet of Steuart—not Loudon—who believed in the providence of a creator having adapted all creatures to their circumstances in the best possible way.] Nevertheless, Matthew claims that Loudon thinks the opposite, or simply mixes Steuart and Loudon up, and criticises this strawman thus:
"Our author's next implied assumption, that a tree produces best timber in a soil and climate natural to it (we suppose by this is meant the soil and climate where the kind of tree is naturally found growing), is, we think, at least exceedingly hypothetical; and, judging from our facts, incorrect. The natural soil and climate of a tree, is often very far from being the soil and climate most suited to its growth, and is only the situation where it has greater power of occupancy, than any other plant whose germ is present." Matthew (1831, p. 302)Compare this with what Loudon said on the effect of culture on plants (that it enlarges all the plant's parts) and what he also said, through Steuart did not quote it, on natural selection (see here), and it will be clear why many contemporaries of Matthew regarded his book as poor scholarship. He was either ignorant or pretended to be ignorant of what the scholars he criticised had truly written. It takes no wonder that such a book should have fallen on many deaf ears and that his idea of natural selection transforming species should have been lost on many a reader.
The following illustrates what has been transported via citation from Loudon over Steuart to Matthew (blue boxes and arrows), and what the reader must assume to be original to Matthew (orange boxes), if (s)he did not know better.
This simplistic image must not be mistaken to indicate plagiarism on Matthew's side. Firstly, Steuart believed in a benign Creator, who adapted all creatures to their circumstances in the best possible way. He therefore had no use for Loudon's idea of natural selection and consequently did nottransport it. Secondly, Matthew (1831) later (not on page 308) combined natural selection with the transformation of species in a way that Loudon did not. He therefore probably regarded his idea as way different from Loudon's run-of-the-mill version. Natural selection was not just in the air, as many unread Darwinists keep repeating, but in print in many places (see here). Matthew naturally saw no need to cite Loudon for this.
To create the impression that Loudon has been a rampant pruner and believed that plants grew best in their natural locations, however, has been poor scholarship.