192 DR. HARLEY ON THE PARASITISM OF THE MISTLETOE. 
theory are much more liable, is thus accounted for. There is perhaps no tree in the 
series more liable to the attacks of the parasite than the Maple, nor one that suffers so 
much from its ravages. 2nd, the nature of the bark, but especially of its periderm. 
The immunity of the Holly, Birch, and Alder is doubtless due to the condition of this 
outer portion of their barks, forming, as it does in them, dry, smooth, unbroken surfaces. 
The Elder rarely grows to a sufficient size to bear the Mistletoe. 
The Poplar, which, as far as my own observations extend, is more liable than either the 
Lime or Ash, appears to be the only exception to the theory; but in this tree there are 
greater predisposing conditions: thus, the nature of the bark is most favourable for the 
attachment and germination of the seed, and the wood is very soft and, from the number, 
. size, and arrangement of the ducts, is easily separable in the direction of the medullary 
rays. The cells composing the medullary rays of the Poplar are almost as large as those 
forming the parenchyma of Viscum; and it may be, that, in plants whose medullary rays 
are small, that one is most liable to the encroachments of parasites whose parenchymatous 
cells approach most nearly in size to those constituting the parenchyma of the parasite. 
. The Mistletoe is but rarely found on Elms and Oaks, and never, I believe, on the 
Horse Chestnut; yet these trees, except as regards their medullary system, appear in all 
other respects, viz. of number, size, aggregation, and condition of bark, to be even more 
exposed to the attacks of the Mistletoe than the Apple and Hawthorn. 
The Hazel and Spanish Chestnut have both a very fine medullary system, and also 
a smooth, unbroken periderm ; and neither of these plants, as far as I am aware, 
spontaneously nourish the Mistletoe. 
From these considerations I think we may conclude, generally, that, other condi- 
tions being equally favourable, the size and number of the medullary rays is the chief 
cause which determines in any given case the attachment of the Mistletoe; and this is 
probably true of all other cases of parasitism. Thus Balanophoracez are found on the 
roots of Maples, Vines, Oaks, Araliaceze*. Cytinaceze occur on Cistus and succulent 
Euphorbias; Rafflesiaceze on Cissus and branches of leguminous plants. Adesmia, 
nourishing Pilostyles, is representedł as having very large medullary rays. 
The positions of the Elder, Holly, Plum, Lilac, &c., in the series probably indicate 
their relative fitness for Mistletoe-grafting. i 
The fibrous structure and deciduous nature of the bark of some trees, e. g. the Vine, 
no doubt prevent the attachment of parasites to them. 
Effects of Mistletoe on the supporting Branch.—The invariable result of the attach- 
ment of the Mistletoe to a branch is the increased development of its tissues, which form 
a swelling around the base of the parasite. It is remarkable that there should be any 
obscurity or difference of opinion as to the mode of the formation of this swelling, since 
its explanation requires little more than a superficial examination. Yet Unger seems to 
have a notion that Viscum exercises some obscure diffusive influence in producing it. 
He says (op. cit. p. 32), “I observed that the germinating seed, as soon as ever it is 
implanted into the nourishing branch, causes already so important a reaction in it, that 
* Dr. J. D. Hooker in Lindley’s * Vegetable Kingdom,’ 3rd edit. p. 90. 
T Unger, op. cit. tab. ii. fig. 3. 
