40 THE FLOWERING PLANT. 



out a little thought on the part of the student, and examination 

 of actual specimens. 



We may now pass to the consideration of bark. A twig can 

 generally be " peeled " with ease, and this is owing to the delicate 

 nature of the cambium ring, which is broken through in the 

 process of peeling. As the wood receive annual additions on its 

 outside from the cambium, so does the bast receive similar addi- 

 tions, the difference being that they are very much thinner and 

 formed on the inside. There is never such a regular appearance 

 of annual rings as in the wood (fig. 7, K). It is evident that the 

 method of increase described must subject the external tissues to 

 a great deal of tension. The epidermis may for some time keep 

 up with the growth in thickness, but sooner or later, in most 

 cases, it bursts and is thrown off. Its place is taken by brown 

 layers of cork cells formed in the following way. A layer of cells 

 in the cortex, i.e., outer part of the ground- tissue, begins to 

 divide actively and forms a layer of meristem, known as cork 

 cambium (fig. 7, I). Its cells, however, are not elongated like 

 those of ordinary cambium. The cells formed on the outside by 

 division of this layer become cork elements. These, as seen in 

 cross-section, are rectangular in shape and arranged in radial 

 rows (cf. p. 31). Their walls are thickened, brown, and com- 

 posed of a substance, suberin, probably allied to cut in (p. 30). 

 They contain no protoplasm. An examination of elder-shoots 

 of different age will illustrate these points. The youngest are 

 bright green, since the chlorophyll-bearing cells of the cortex 

 can be seen through the transparent epidermis, but older ones 

 are brown, owing to the formation of cork. This is not very 

 transparent, and so does not permit the green cortical tissue to 

 be clearly seen from the outside. A cross-section will show the 

 relation of the different parts. If this is made through a branch 

 about half an inch thick, the following points can be easily made 

 out by the eye alone. A large part of the interior will be occu- 

 pied by the bright, white, spongy pith, the colour of which is due 

 to the fact that the cells are dead and contain air instead of pro- 

 toplasm. Then follows a whitish-brown band of wood, in which 

 medullary rays are plainly seen. The cambium forms an ex- 

 tremely narrow band, the position of which is shown by the fact 

 that the outer part of the stem can be peeled off at this point. 

 Next comes a narrow greenish-white zone, the bast, and this is 

 succeeded by a narrower dark-green portion, the chlorophyll- 

 bearing cortex. This is invested by a thin papery layer of 

 whitish-brown cork, which can easily be detached, owing to the 

 delicate nature of the cork cambium. That it is subjected to 

 tension may be seen by the presence of numerous little fissures, 



