CELLULAR STRUCTURE OF TREES. 



85 



not be cylindrical, and that the layers of wood will not be in perfect zones. So also 

 when this disturbance is restricted to a portion only of one side, fhere will be no 

 growth at that part, and in process of time a depression in the stem will result, giving 

 it a furrowed appearance. At a still later period, assuming that the like causes exist, 

 this furrow will become deeper, but at the same time it will be narrower; for the woody 

 fibre, as it passes down on either side, will find little resistance in that direction, and 

 will push into the furrow and lessen its size. At the same time the bark will also 

 increase in thickness, and in process of time the original furrow will have disappeared. 

 A section of such a stem would show a triangular interval in the circumference 

 of the trunk, which would either be vacant or filled up with layers of bark. If, 

 whilst these changes are proceeding, others of a similar character were met with in 

 other parts of the circumference, the section, instead of exhibiting a circular outline, 

 would greatly resemble the figure of the stellate cell (page 11). These are the expla- 

 nations of a great variety of twining stems growing in hot climates, and which are 

 angular, or present a cruciate appearance on section. 



An interesting modification, and one very nearly allied to the above, is that in which 

 the medullary rays increase in thickness so greatly as not only to be mere lines, giving 

 a grain to the wood, but large wedge-shaped blocks between alternate masses of wood. 

 This is not remarkable, when we remember that at every moment of growth there are 

 two processes going on, one the cellular or horizontal, and the other the woody or 

 vertical ; and it is no more a matter of surprise that nature should occasionally increase 

 the one at the expense of the other, than that she should rigidly adhere to the rule 

 which she has laid down ; for both the rule and the exception are alike wonderful and 

 inexplicable. Such exceptions, greatly varied, but yet for the most part originating in 

 the " Wood," or cellular structure of the stem, are by no me ins uncommon. 



The general configuration of exo- 

 genous stems is conical, the circum- 

 ference being, for the most part, cir- 

 cular, and the base much larger than 

 the apex, or the free terminal part of 

 the stem. This necessarily results 

 from the remarks which we have 

 made on the production of wood ; for 

 it is manifest, that if the wood be a 

 product of the leaves, and the number 

 of leaves on the tree increases from 

 above downwards, the quantity of 

 wood deposited will be greater below 

 than above. The apparent exceptions 

 to this rule are in such fast-growing 

 trees as grow in the midst of a dense 

 wood, where the light reaches them 





e d c d cd 



Fig. 142, showing the component parts of a stem in 



the fourth year of growth, 

 only at the top. bucn trees run up ot A, a part of a transverse section. B, a perpendicular 



nearly even diameter, and without Bother h6 PartS f GaCh arranged accuratel y over 

 a branch, until more than two-thirds a, the pith; b, the surrounding medullary sheath; 



c and d layers of wood and bothrenchym intermin- 

 gled. The open-work in A shows the position and 

 the extent of bothrenchym more clearly ; e, the bark 



of their entire height has been at- 

 tained ; but from the point where 



branches arise, the conical figure may readily be traced. The common asparagus 



