Helrospectivc Criticism, 197 



is tlie following : — The sap in its ascent in the stem becomes depfiveJ of 

 some of its constituents, more especially of its aqueous part; this depriv-f 

 ation is effected by the vital principle oi the plant decomposing the aqueous 

 parts, and assimilating the resulting gases to its own constituents. As the 

 assimilation takes place a partial vacuum is formed by the change of gases 

 to a solid form, and this vacuum is immediately filled with sap rushing, 

 into it, according to the well-known law of the tendency of fluids to rush 

 into any cavity deprived of the presence of air." 



This theory would be very convincing, but for one circumstance ; 

 namely, for once, that is to say, for the first time, the sap must rise into 

 the upper part of the tree, there to be " deprived of some of its con- 

 stituents," owing to some other cause than the one proposed in the theory, 

 of H. : when once it has risen then H,'s theory would account for its 

 continuing to ascend. 



What I want H., therefore, to explain is, what is the cause why the sap 

 rises in the first instance into the upper part of the stem ? What impulse 

 forces or drives it up primarily, in order that, when it has ascended, the 

 process of assimilation, &c., vacuum, and renewed supply, may take place 

 in th2 manner his theory supposes. 



When H. has given us that information, I shall rest satisfied. This, 

 however, cruel as it may be, will most likely throw H.'s theory overboard ; 

 as it occurs to me that the cause which produces the first rise of the sajj 

 will suffice to account for its subsequent ascent. In plain English, although 

 I fully admit the ingenuity of H,'s theory, I feel confident it cannot be the 

 true one; as the sap must have ascended once before his system can come 

 into play. Yours, &c. — Thomas Thompson. Hully Nov. 9. 1831. 



Relative Scarcity of certain Species of, British Plants y Changes in their Ha- 

 bitatSy and Introduction of Exotic Species in'o British Localites. — Sir, Tlie 

 remarks of Mr. Lees (Vol. IV. p. 437.) relative to my notice (Vol. IV. p. 28.) 

 of /Vis tuberosa as a native of Ireland, induce me to add a word or two 

 of explanation on that subject. Mr. Lees is a practical and intelligent 

 botanist, and I do not wonder that he should be a little sceptical on the 

 point in question. All that I positively contend for is, that this Vvis has 

 as strong claims to be considered a native of Ireland as many other plants 

 which are admitted into our indigenous list. The plant is undoubtedly of 

 exotic mien and aspect; and the circumstance of its being met with near a 

 large town, and in the vicinity of an old ruin, which Mr. Lees justly con- 

 siders as casting a degree of doubt on its claims as a genuine native, had 

 previously occm'red to my own mind, and created the same suspicions. 

 Should these remarks meet the e3'e of Mr. Drummond, to whom I am 

 indebted for having introduced me to the Irish habitat of /^ris tuberosa, he 

 possibly might be able to throw some additional light on the subject. I 

 entirely concur in all that Mr. Lees advances about the botanical or hor- 

 ticultural propensities of the monks in olden time ; and admit the propriety 

 of his remark, that " v/hen we find any rare plant on, or in the neighbour- 

 hood of, a ruin, we ought to be very suspicious whether the plant is not a 

 naturalisation, rather than a real denizen." Our Hibernian /Vis, accord- 

 ingl}', may possibly have been introduced. I do not think, however, that 

 its introduction has been a fraudulent one, or that it is of recent date ; and 

 at all events the species has nov/ become naturalised, and therefore, as I 

 have already remarked, has as fair a claim as many other plants to a place 

 in our British flora. There are, I believe, those who contend that ^'rbutus 

 C/'nedo, which grows so abundantly on the rocks and islands of the romantic 

 Lake of Killarney, is no original native, but has been introduced to that 

 station by the monks of Mucruss. Be this as it may, whoever has seen the 

 shrub luxuriating as it does in the wild distiict of GlyngarafF, near Bantrj' 

 Bay, can hardly doubt its being truly indigenous. 



o 3 



