FORMATION OF TISSUES FROM GUM. 209 



exudation is thrown out from the cut edges ; and by the conver- 

 sion of this into solid tissue, the wound is gradually healed. If 

 a .complete ring be cut away from the bark, this exudation will 

 be much the greatest on the upper side, showing that it comes 

 from the descending sap ; but it is not altogether confined to that 

 edge, since a portion of the descending current, having been 

 carried by the medullary rays into the interior of the stem, is not 

 checked by this interruption to its flow through the bark. Thus 

 we perceive that, although there is not in Plants, as in Animals, 

 a regular continuous circulation of nutritious fluid, that which 

 has once passed through the system of the latter, being impelled 

 again through its vessels, after having undergone the necessary 

 purification, Nature has provided for the reparation of their 

 wounds in the most advantageous manner. 



331. From this form of Gum it would appear, that the ma- 

 terials of the tissues are produced ; but those by which woody 

 fibre is consolidated, are not the same in chemical constitution, 

 containing a larger proportion of carbon. And thus we see why 

 it should be peculiarly necessary for the production of firm wood, 

 that the leaves should be exposed to the full influence of light, 

 by which alone the proper amount of carbon can be introduced 

 into the system. As already stated, whilst cellular tissue in- 

 creases in every direction, woody fibres seem to grow almost 

 exclusively downwards. They may be traced gradually descend- 

 ing from the leaves, in which they always originate, just as the 

 roots make their way through the earth. They pass down in 

 the space between the bark and wood, at the time the cambium 

 is there ; and this fluid probably contains the materials for both 

 tissues. If an obstacle intervene, as, for example, a branch 

 passing off from the stem, they do not stop in consequence of it ; 

 but they separate to one side and the other, and re-unite below, 

 just as a bundle of roots would have done. These fibres, being 

 intermixed with the cellular tissue produced by the cambium, com- 

 pose the new layers of wood and bark, of which a new one is 

 formed every year ; and it is in this way, that those additions are 

 made to the quantity of solid matter contained in the stem, \vhich 

 the supply of descending sap is principally intended to furnish. 



