ON THE STEM OF TREES. 



339 



the cylinders at night and in cold weather, they would 

 equally have the effect required ; that of supplying by their 

 pressure the zoant of the sun's rays. The wood vessels 

 are far more simply made than those of the bark ; they are 

 very narrow cylinders; and the last two rows next to 

 the circle of life are sap vessels covered by the spiral ones. 

 The horse chesnut has three or four rows, and they appear 

 to be in quantity according to the size of the leaves. It is 

 indeed difficult to ascertain them exactly even in the solar 

 microscope, as it is in unwinding them alone they can be 

 knozvn; and their extreme fineness confuses. This has Spiral vessels 

 however caused the spiral vessels to be taken for sap vessels. n sap 

 It was a great pleasure to me to find, that neither Mr. 

 Knight nor Mirbel was of this opinion. I believe there can 

 be no doubt, that they are solid strings, and hold no liquid. 

 When wood is very aged, it grows so compact, that it is dif- 

 ficult without preparation to see the open mouths of the 

 vessels. The wood should then be cut in thin slices, and All cut with 

 laid in a very dry place ; and it is wonderful how this !WJlJ!Lv£32w 

 stretch the upper end of the cylinders; but fresh cut speci- 

 mens, if examined directly, will almost always be suf- 

 ficiently visible. If much magnified, and cut longitudinally, 

 it is truly wonderful to see the effect of light and heat on the 

 wood vessels ; how immediately on turning the light on the 

 glass, the flow of sap is accelerated, and with what perfect 

 ease it runs up vessels so diminutive, that to measure them 

 is almost impossible. Is it not most wonderful to consider 

 the force necessary to carry up this sap, when the vessels 

 are formed of a substance so thin, so transparent, that it 

 would appear impossible to confine a liquid within it; and 

 yet that, without being worn out by friction, it will bear 

 this force exerted against it, for eighty years together, 

 without showing any signs of decay, a term which many 

 trees will sustain ? This indeed proclaims its author, and 

 should make the atheist fall down and worship. A few of 

 the wood vessels are separated, and run with the spiral 

 vessels as nourishing vessels to each leaf, as I have shown 

 in my last ; but this diverts but little of the sap from its 

 chief current, which flows on; its last purpose being to 

 form the stamen, and the curious powder that apertains 



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