[KIRSCH] ORIGIN AND DEVELOPMENT OF RESIN CANALS 63 
of the tumor, where the canals also reach their greatest diameter. 
From the middle of the tumor towards its apex the number of canals 
decreases, the greater number of them ending at its upper end.” 
In both of these cases, the largest number of canals is found in the 
region where the food was most abundant, thus bearing out our asser- 
tions. Examples of this relation of the number of canals to the vigor 
of growth in the forms where they occur could be multiplied without 
number but to no purpose, as the above suffice to bear out the point 
under discussion. 
Moreover, in forms where canals normally occur, a great diminu- 
tion or even a complete absence of those structures would be expected 
in specimens exhibiting specially weak growth, whilst where they occur 
only sporadically or not at all in normal growth, they would be found 
most numerous and best developed in such examples as grew under 
conditions entailing an abnormal supply of food. This point also is 
borne out by all ‘recorded observations. Thus Mayr found that the 
number of canals in the aerial portions of the seedlings of Picea excelsa 
and Larix europæa varied with the vigor of growth. He also observed 
that the most delicate roots possessed no resin canals. Again, he states 
that the distribution of the passages in the wood was an extremely 
varying one, some growth rings possessing only a few canals, others 
numberless ones (Zahllose).’”’ 
The writer, in examining the growth rings of different Coniferæ, 
found many species of Pinus, Picea, Larix, and Sequoia showing no 
canals in some of the rings, whilst they were fairly numerous in others, 
the former being usually the case where there was evidence of weak 
growth. In a section from an exceptionally weak twig of Pinus strobus, 
which, although exhibiting four growth rings, was only 1-3 mm. in. 
diameter, no canals at all were present in the first two years of growth, 
some, however, appearing in the subsequent growth of the wood. Those 
cases where resin canals are normally absent, but arise in response to 
unusual conditions will be dealt with at the end of this section of the 
paper. 
Before proceeding to the discussion of the function of the wood 
parenchyma aggregates which usually enclose an intercellular passage, 
it will be well to show, in addition to the various details represented in 
the figures, that the structure of the cells composing these vertical 
ageregates is essentially the same as that of the cells of the medullary 
rays in the same forms. 
De Bary (5, 486) says, “The parenchyma of the medullary rays 
consists, in the great majority of secondary woods, of cells which have 
essentially the same properties as the parenchyma of the bundles in 
the same plant, without being exactly similar in every point.” 
