ENERGY IN PLANT-CELLS. 



181 



Fig. 1. 



new layers the tree has crowded off the block, drawing the head of the 

 spike directly through the pine wood ; that is, new material has been 

 thrust in between the wood of the tree on 

 the one hand and the block on the other, 

 until the block has been fairly wedged 

 from its place. Now, it is asserted on 

 excellent authority that the force neces- 

 sary to accomplish this result amounts to 

 a pressure of about thirty pounds to the 

 square inch ; i. e., the forces of growth in 

 a soft maple are capable of exerting in all 

 directions a force of thirty pounds to the 

 square inch. Now, these results may seem 

 somewhat surprising, but our surprise is 

 in no degree lessened when we begin to 

 study the machinery by which this energy 

 is exerted. If we could make a cross-sec- 

 tion of one of the trees in question, we 

 should find by far the greater part of 

 the tree in a condition of nearly absolute 

 fixity, incapable of enlargement in any 

 direction. Outside is the bark, likewise 

 largely incapable of exerting force, most 

 of the cells having long since yielded up their living matter. Only on 

 the line of division separating bark from wood do we find a structure 

 whose cells are capable of life, growth, and multiplication. This 

 structure is so thin that only the finest line would be needed for its 

 delineation, were we to draw the 

 whole section, natural size (Fig. 2). 

 Furthermore, this layer is made of 

 cells whose walls are exceedingly 

 delicate and thin. So much more 

 feeble, in this regard, are the cells 

 here than on either side, that this 

 layer, the cambium, is the line of 

 separation when, in the growing 

 season, you easily strip the green 

 bark from the wood. The energy, then, which we have estimated, 

 must finally rest upon these thin-walled, delicate cells. Not only is 

 this true, but. we may also easily conclude that all the pressure by 

 which the cleat is wedged from the tree must come from the growth 

 and multiplication of the same diminutive organisms. It is plain here 

 that the force concerned is not capillary, for that is certainly as active 

 in the woody parts of the tree as in the cambium, there producing no 

 expansion whatever. Neither does it seem that the energy expended 

 must be attributed to osmosis, although the cell may be by construc- 



Fig. 2.— Ckoss-Secttox of ax Exogenous 

 Stem. 



