ANATOMY OF TISSUES. 141 
a free communication between the air outside and the chloro- 
phyll-holding cells within. At the close of the summer, when 
the time for the winter rest draws near, this opening is usually 
closed by the last layers of the phellogen growth. That is, 
these layers remain a continuous collection of cells and their 
walls become suberized. In the spring time, the phellogen 
renews its activity, and the new cells, pushing up as before, 
break these layers apart and the opening is established anew. 
(Fig. 63.) 
On stems and roots where the first periderm is replaced by 
bark, the communication necessary for breathing is sometimes 
Se = 
stece ee . 
Fia. 63. 
Lenticel from a branch of Gleditschia, with several closing layers, v. epidermis. v' 
closing layers broken off. J rejuvenating layer. / filling cells. p parenchyma. 
s schlerenchyma. 
made by the fissures and rifts of the bark extending through the 
corky covering ; and in other cases by the growth of lenticels 
originating with no reference to the place of the original stomata. 
These lenticels originate, in some instances, under the fissures, 
so they open into them; in others they occur under the ridges 
and extend through the entire thickness of the bark and open 
on the ridges. The central portion of woody tissue in stems of 
considerable age is known as heart wood. Its elements are dead 
and its color is usually darker than that of the surrounding cyl- 
inder, which is called splint wood. This latter is that portion 
of the wood which conducts the water currents upward, and 
