No. 406.] GALL OF THE MONTEREY PINE. 809 
unsatisfactory, answer can be given. And this must be based 
on the all too slight evidence which lies before us. 
There is no indication that the hypertrophy is either caused 
or affected by any substance deposited with the eggs. 
Extensive experimentation would be necessary to prove what 
the effect would be of a foreign body, living or lifeless, in con- 
tact with such plant tissues; or, again, how much the waste 
matter given off from a living body would affect the tissues. 
There is some evidence, although negative, touching the former 
in the cases cited above, where the eggs of the gallfly were 
placed either between the scales or between the inside scale 
and near the tip of the leaf. In neither of these cases did the 
larvee mature, and the reason for this must be that the cells 
which touched the young larvae were not such as could easily 
give up their contents to nourish the parasite. In either case, 
however, they hatched into larva, and we may believe that, 
although no food may have passed from the cells of the host to 
the parasite, yet the plant tissues may have been mechanically 
irritated by the mere presence of a living foreign body. That 
the leaf epidermis was thickened would appear to make no 
difference if the plant tissues in question were stimulated to 
abnormal growth, mostly by this mechanical irritation, because 
cases are not wanting among plants! in which epidermal cells, 
although strongly cutinized, have been rejuvenated and have 
been caused to perform functions other than normal, or at least 
usual, and this, too, mainly from external mechanical stimu- 
lation. It is highly probable that the mere presence of the 
parasite does, to some extent, in the case of this gall, as in 
others, stimulate the living plant tissues. 
Briefly it appears that the immediate and principal cause of 
the hypertrophy is the response on the part of certain plant 
tissues to the parasite’s demand for food. This is indicated 
mainly by the gradual enlargement of the cells surrounding the 
parasite, in a manner which corresponds to its growth, and 
also by the unusual amount of food material which these cells. 
contain. 
l Peirce, G. J. On the Structure of the Haustoria of Some Phanerogamic 
Parasites, Annals of Botany, vol. vii (1893), No. 27, p- 295 
