266 ANATOMY OF STEMS. 



portions of insulated bark, on other branches of the 

 same age, no apparent increase in the size of the 

 wood was discoverab'e."* 



These experiments explain the reason why trees 

 and shrubs having their leaves destroyed by cater- 

 pillars form scarcely any new wood in that season ; 

 and, indeed, every one who has ever pruned a tree, 

 or shortened a growing twig, must have observed that 

 the part above the last leaf always shrivels and dies, 

 while all below it continues to live and increase in di- 

 ameter. 



The Medullary Sheath. If we proceed with 

 the examination of the shoot of the Horse Chesnut, 

 as before, and scoop out the pith from the ligenous 

 cylinder that encloses it, we shall perceive that this 

 is lined with a thin green layer or coating ; which, to 

 the unassisted eye, appears to resemble in its struc- 

 ture rather the cellular integument of the bark than 

 any part of the surrounding wood. This is the Me- 

 dullary Sheath of Mirbel and the French Botanists. 

 It is readily distinguished, in either a transverse or a 

 longitudinal section of many stems, by its green color, 

 which appears deeper as contrasted with the dead 

 white of the pith which it surrounds ; but it is also 

 easily traced in the succulent dicotyledonous stem as 

 soon as it is evolved from the seed, separating the 

 pith from its herbaceous investiture. 



When viewed under the microscope, the Medulla- 

 ry Sheath appears to be composed of a cellular sub- 

 stance, in which are imbedded longitudinal layers of 

 spiral tubes. The cells of the Medullary Sheath are 

 narrow and oblong ; and, therefore, when it is not 

 colored, it is scarcely distinguishable from the wood, 



* Philos. Transact. 1801. P. I. p. 2, p. 335. 



