OR, PLAIN TEACHING. 



199 



that formed the previous year. 

 The pithy 1, occupies the centre 

 of the stem, then follow the 

 annual layers of woody fibre, 2, 2 ; 

 then the channel which in spring 

 is lubricated with cambium, or 

 sap, 3 ; and, lastly, the bark, 4. 



2 



Illllill 



3 



1-4 



559. 



The sap moistens the surfaces of 

 the wood and bark, and enables 

 the new woody matter, formed 

 by the leaves, to descend more 

 freely. This substance disap- 

 pears every spring after the com- 

 plete formation of the wood, 

 which then adheres firmly to the 

 bark ; but it reappears whenever 

 the plant is again called into 

 growth ; and it occurs twice a 

 year in those plants which shoot 

 a second time. 



From the period that the sap is absorbed by 

 the roots to the time that the various secretions 

 of the plants are elaborated, it is in a state of 

 constant motion. This motion is called the 

 circulation of the sap, but it is not intended to 

 convey by the term " circulation " the idea that 

 the movement is similar to that of the blood of 

 animals, which is constantly sent out t from and 

 returned to a central point. The motions of 

 the fluid in plants are of two kinds, general 

 and special. The general motions of the sap 

 are those of ascent and descent, both of which 

 may be rendered apparent by cutting through 

 the trunk of a tree, when not only the cut sur- 

 face below will present an exudation of juice in 

 its ascending course, but the cut surface above 

 will present fluid that is descending. The exist- 

 ence and amount of the fluids thus circulating 

 in plants has been demonstrated by an inge- 

 nious apparatus invented by M. Biot. By means 

 of a groove in the lower surface of a hole in the 

 stem of a tree, and a little trough applied to the 

 upper surface, he measured the amount of as- 

 cending and descending juices, and also the in- 

 fluence of external circumstances upon the flow 

 of the fluid. It was observed by M. Biot that 



the descending current is more dense and sac- 

 charine than the ascending, although this is 

 subject to slight alterations during ♦•ain. Light 

 is the principal agent in modifying the flow. 

 Mild weather facilitates the ascent of the sap ; 

 and a sudden cold, by contracting the tree, 

 appears to promote its descent. If the cold 

 continues, the ground hardens, and the sap 

 again ascends. If a thaw succeeds a frost, the 

 roots require replenishing, and a downward 

 current is established. The ascent of the sap, 

 which is so strong in spring, ceases when the 

 leaves are fully expanded. After the middle of 

 summer, the sun's rays have less power, the 

 leaves also are obstructed by the deposition of 

 secretions, the whole tree attains a state of 

 plethora, and there is then an increase of the 

 descending juices.* 



The age of trees has been cal- 

 culated by estimating the number 

 of wood-rings found in the sec- 

 tion of a trunk — each ring being 

 regarded as the growth of a year. 

 Although in some respects liable 

 to error, from the irregular 

 growth of trees, and the great 

 compression of the original rings, 

 an approximation to accuracy is 

 nevertheless obtained. The ages 

 of trees have been calculated, 

 from the palm of 300 years, to the 

 dragon' 's blood tree of 6000 year»s, 

 which carries us back to the days 

 of Adam ! 



Besides the motion of the sap 

 from the root to the leaves, and 

 from the latter to the root again, 

 there exists a communication be* 

 tween the pith and the bark, by 

 means of medullary rays. Thus 

 each trunk has two systems — the 

 pith, 1, me- 5 

 dullary rays, 

 5, and bark, 

 4,comprising 

 the horizontal 

 system, . ope- 

 rating across 

 the diameter, 

 and the wood, 

 with its ves- 560 - 



• Penny Cyclopaedia. 



