ON STEMS. 51 



have happened in the year 1709, which was remarkable 

 for the severity of the frost. 



Emily. But since the layers of wood grow with so 

 much regularity, whence come those knots and waving 

 lines, which constitute the beauty of polished wood ? 



Mrs. B. If the sap, in rising through the young wood, 

 meets with any casual obstruction to its passage, it natu- 

 rally accumulates in that spot, and forms what is called 

 a knot. This consists of distended vessels, containing 

 a magazine of food, which gives birth to a germ or shoot ; 

 but it frequently happens that before this germ has attain- 

 ed strength to force its way through the bark into the 

 open air, a new layer of wood rises over and encloses it. 

 Sometimes it is only temporarily buried ; and the follow- 

 ing season it acquires sufficient vigor to break through its 

 prison. Thus, if the shoot go on annually forcing its 

 way through the wooden wall which rises up to oppose 

 its progress till it reaches the surface of the stem, it be- 

 comes the origin of an external shoot or branch. If, on 

 the contrary, it is exhausted by this series of struggles, 

 it perishes ; and leaves, in memorial of its efforts, the 

 knots, waves, and streaks, which embellish its tomb. 

 This shoot which had increased in size whilst traversing 

 the several layers of wood, as soon as it grows externally, 

 diminishes as it protrudes in the air, being thickest at the 

 stem, and tapering towards its extremity ; so that a shoot, 

 if traced from its origin, exhibits the form of a double 

 cone, the base of which is at the surface of the stem. 



Emily. But whence did this shoot derive its origin ? 

 The accumulation of sap can merely favor its growth, but 

 cannot have given it existence. 



Mrs. B. This is a question not very easily answered ; 

 but the opinion most prevalent among botanists is, that 

 germs or latent shoots exist throughout the stems and 

 branches of plants, and that those only are brought into a 

 state of active vegetation which are fully supplied with food. 



Caroline. Do the stems and branches of exogenous 

 plants grow like their roots, merely at their extremities ? 



254. What question does Emily ask concerning knots in woodl 255. 

 What is the consequence if the sap in rising is obstructed 1 ? 256 What 

 gives birth to a shoot or branch 1 ? 257. What sometimes will prevent 

 the inward germ from becoming an external branch'? 258. What is its 

 form when it grows externally 1 ? 259. What question does Emily ask 

 concerning the origin of this shoof? 260. What is the answer of Mrs . B*? 



