36 THE BARK. 



been most conclusively proved by Hugo Muhl, and Professor 

 linger, that there is no necessary connection between the 

 lenticell and root ; and moreover that adventitious roots are 

 made to spring from parts where a lenticel was never seen. 

 The student would be little benefited by the numerous views 

 taken of these products when in reality they probably have 

 no function at all, as lenticels, or at most serve only as pas- 

 sages for air beneath the cuticle. 



Section 2. — The Bark, 



41. The hark, which lies immediately beneath the cuticle, 

 consists of an external layer of green spongy substance, as 

 seen in fig. 15, &, and an internal layer of fibres interlaced 

 with each other, and the interstices filled with cellular tis- 

 sue as seen in fig. 15, c. The first is called the Cellular 

 Integument^ and the second the Liber. The cuticle, cellular 

 integument, and liber, may be very readily examined in a 

 branch of the cherry of one year's growth. The cuticle will 

 readily peal off, tearing transversely. The cellular integu- 

 ment may then be easily separated from the subjacent liber. 

 The two layers of the bark are each formed every year, and 

 of course the thick bark of old trees is made up of alternate 

 layers of cellular integument and liber. From the enlarge- 

 ment of the stem, and the internal formation of bark, the 

 outer layers become distended and broken, and thereby pro- 

 duce the rugged appearance of some old trees, and the annual 

 peeling oflf of the bark of the sycamore, which, from the 

 slight cohesion of the different layers, falls oflT as. soon 

 as broken, not forming the longitudinal ridges like those of 

 the oak and pine. An examination of the bark of an old 

 pine will give the student a correct idea of the eflfects produ- 

 ced by the constant enlargement of the stem, and the yearly 

 deposition of the bark. Instead of finding regular layers of 

 bark, he will observe, that the bark consists of irregular plates, 

 each being composed of two lamena, one thin and membra- 

 nous, the other thicker and of more consistence ; but in most 

 cases, exactly corresponding to each other in form. These 

 two lamina correspond to the cellar integument and liber, 

 and the cause of their existing in irregular plates instead of 

 layers, is their distension until the layers become torn, and 

 the parts separated. It not unfrequently entirely decays, 

 thus showing that it is necessary only in the early stages of 

 the plant. ^ 



