146 Vegetable Physiology. 



very plainly marked. But in most cases this change is more 

 gradual. It is produced by the consolidation of the interior 

 portion, whose tissue becomes so pressed together, and its cells 

 so filled up by different secretions, as to prevent the passage of 

 any fluid through it. It then becomes of no use to the system 

 except to give it strength and durability, since it is through the 

 new layers, or sap-wood, that the passage of the sap is effected 

 As the pith and the inner layers thus gradually become unfit 

 for their original use, and as in the outer portion alone the pro- 

 cesses of vegetation go on, the former may be removed without 

 injuring the latter ; and this operation is often produced by 

 natural decay, which destroys the heart of an old tree, leaving 

 the outside only a shell, though still capable of producing 

 buds and branches. 



A variety in the structure of exogenous stems, is that before 

 mentioned as belonging to the Pines and Firs. Their annual 

 layers are generally marked with great regularity and distinct- 

 ness, especially in those which are natives of cold or temperate 

 climates, where the process of vegetation is quite interrupted 

 by cold after the formation of each layer, while in warmer 

 regions they pass into one another more gradually. In the 

 latter case, too, the thickness of the layers is nearly uniform, 

 while in the former, it differs according as the seasons have 

 been favorable or otherwise. In examining fossil plants, these 

 facts are important and interesting, as will be seen. 



In a circle around the pith, and between it and the first 

 layer of wood, is arranged a series of spiral vessels, which 

 seldom occur in any other part of an exogenous stem. This 

 forms the medullary sheath. 



The outermost coat, enclosing the wood, is the bark. This 

 is also formed in regular layers, from the interior, which are so 

 much thinner than those of the wood as not to be so easily 

 distinguished. The oldest layers of course are on the out- 

 side, and become gradually lost, by decay, or by falling off, 

 so that it is in general impossible to trace the same number as 

 in the wood, although they are formed at the same time. As 

 each new layer of wood is formed on the outside of the pre- 

 vious one, and therefore at the circle where it is in contact with 

 the bark, and as the new layer of bark is added to the inside of 



