14 A MANUAL OF THE CONIFERS. 



kind according to the soil and situation, it is also influenced by the 

 state of the season, being greater or less according as the temperature 

 is higher or lower than the average mean. 



The following known physiological conditions go a great way towards 

 explaining the cause of this rapid increase in the stems of Coniferous 

 trees. The tubes and channels through which the sap circulates have, 

 comparatively, a greater diameter than those of most other trees; they 

 therefore admit of and even require an abundant supply of fluid to 

 fill them; they obtain this supply by means of the roots, which 

 possess an enormous absorbent power. These roots, it must bo 

 remembered, are generally near the surface, so that as soon as tho^ 

 warm days of spring and summer begin to put the sap in motion 

 in the terminal branchlets and parts most exposed to the sun's 

 influence, and also to raise the temperature of the soil sufficiently 

 to excite the rootlets into action, which in most of the species is 

 very energetic, and produces a corresponding rapidity in the circulation 

 of the sap and the formation of new tissues,* the growth of the 

 tree proceeds apace, the stem not only increases in height but also 

 in diameter. The deposition of the insoluble matters taken up by 

 the roots, soon choke up the older channels of circulation, and 

 the successive annual layers or rings pressing closely around those 

 previously formed, the trunk becomes consolidated. Some of the 

 members of the Fir and Pine tribe, and notably the gigantic Sequoias 

 of California and the Common Yew continue to grow for centuries, 

 adding yearly ring after ring to the circumference of their stems, 

 building up vast pillars of wood' that are the wonder of all who 

 behold them. 



Theoretically, the trunks of Coniferous trees, like other Exogenous 

 stems, might increase in size and height indefinitely, were there no 

 counteracting causes at work to check and finally to arrest the 

 progress; but such sooner or later are sure to arise, and among the, 

 principal is undoubtedly the gradual exhaustion of the soil in which 

 the tree is growing. The functions of the various organs become 

 enfeebled by age, as they do in the animal frame, although the 

 period of the life of the one is in most instances immensely prolonged 

 compared with that of the other, so that the cause of decay is so 

 much the slower in its action. The vigour with which Coniferous 

 trees increase in size during the earlier period of their existence is 

 sensibly diminished in process of time, till at length the counteracting 

 causes balance the growing power; the tree has then reached its full 

 maturity; the period of decay sets in which is never permanently 



* Every one who has made an incision through the bark of the Common Larch, the 

 Spruce Fir, or the Scotch Pine especially at any time during the growing season, is aware 

 of the very rapid flow of sap from the wound. Advantage fs taken of tlis rapid flow to 

 procure the turpentine of commerce from the species that yield it by making incision in 

 the bark, and keeping the wound open, J •- 8 ™' ,M ™ iu 



