[KIRSCH] ORIGIN AND DEVELOPMENT OF RESIN CANALS 77 
canals in normal growth wasted much of its substance, the loss of these 
canals in the progeny tending to economy; why, then, are the very 
rings in which the resin canals appear traumatically ‘unusually thick’? 
If the plant economized by dispensing with its ‘ancestral’ resin canals, 
it would do so for the purpose of being able to manufacture elements 
more useful to it e.g., tracheids; while in this case the very growth 
rings which ‘revert’ to the less economical habits of the hypothetical 
ancestors how a marked increase in the amount of tissue formed. 
Jeffrey also observed that the “ Horizontal traumatic canals may 
pass outward from a healed wound through many annual rings” (12, 
325). He found that some of the horizontal ducts passed through over 
seventy annual rings before tapering off and ending blindly. 
According to Jeffrey’s theory there is no explanation why the 
horizontal resin canals in this case should keep on “reverting” through 
a great many growth rings, while the vertical ones are confined to only 
one or two growth rings. Our theory, however, affords a very rational 
explanation of this phenomenon as can be seen from the following. 
If the rapid growth consequent on wounding is not confined mainly 
to an increased growth in diameter, as is the case in the majority of 
traumatic reactions, but is also exhibited in a more vigorous growth 
in length of the stem, it becomes possible for fusiform rays containing 
a canal to be formed. Now the stimulus which causes the formation 
of traumatic resin canals in the vertical plane is usually present only 
during the first year after the wounding of the tree; although, in 
exceptional cases, it may persist in a modified degree during two or 
three years subsequent to wounding. Therefore these vertical canals 
are, as a rule, formed only in the first growth ring after the wounding 
has taken place. Moreover, since the vertical resin passages in the 
Coniferæ end in the growth ring in which they are formed, there is no 
possibility for the ones formed in the wood of the first year after the 
wounding to pass into the wood of subsequent years. Vertical resin 
ducts can be formed in subsequent years only if the stimulus due to 
a former wound is still strong enough to call them forth or if the tree 
has been injured anew. The position of the horizontal resin canals, on 
the other hand, renders their continued growth in a radial direction 
independent of the stimuli which initiated their formation; their 
extension through any of the growth rings laid down subsequent to the 
injury depending on the general conditions in the tree. The fusiform 
rays in which they are situated afford a larger channel for the elaborated 
foodstuffs from the bast than do the uniseriate rays, and so enough 
food is brought to this part of the growing tissue to enable the con- 
tinuance of the fusiiorm ray and hence of the canal. The passage would 
thus keep on extending through many growth rings until a very poor year 
