UU 
(44. — Structural and Physiological Botany. i 
~which, when looked at vertically, have a regular polygonal or 
stellate appearance. It has in general more vital power, and 
hee wi EPS Bay ce Any pas a aa te Werieens We hs Mee ty pS kh 
1‘ j Pill ~ “\ SO athe Tr) *» a? > bs Ce Saae Avid 
1 NS 
~ \ z vf ~—' 
» se ~: tay A A 
i ARs NT CoB gE 
t 
ie ti Os 
Sah aoa 
Periderm consists of tabular cells with thicker walls, 
Fic. 66.—Transverse section through the bark of the juniper ; B B bast-cells ; 
m m medullary rays; & layer of cork formed of thin-walled cells parallel 
to the circumference of the stem. (Xx 370.) | 
is firmer, less elastic, and more leathery than true cork. 
When it is several layers in thickness, thick and _ thin- 
walled layers often alternate (Fig. 67), causing frequently a 
peeling off, as in the birch and cherry ; in the beech and i 
silver fir, on the contrary, the periderm does not peel Off. ae 
Cork is the true healing-tissue of plants, and as such is of great 
importance. In the fresh surfaces of wounds, the uppermost layers of 
‘cells as a rule dry up, while in the tissue beneath them, still full of sap, 
new cells are formed which are the origin of the cork. A very good 
example of the importance of this production of cork is afforded by the 
cochineal-cactus. This plant is multiplied by cuttings, which would: 
decay at once if they were set in the ground with the surfaces of the 
wound fresh. The shoots which are intended as cuttings are therefore 
first laid for about four weeks in a dry place exposed to the sun, in 
4 
