194 PLANT ANATOMY. 
Even on leaves a local formation of cork occasionally occurs’ 
(Eucalyptus, Fig. 128, k). 
The physiological function of cork is to protect the tissues 
lying beneath it from too great evaporation and from mechani- 
cal injury. The former function is presented in a very striking 
manner by the potato, in which the cork layers, from five to ten 
in number, cause the succulence of the entire inner tissue to be 
retained for a long time undiminished. 
On wounds, the so-called wound cork is frequently formed 
(Fig. 109, a). This produces, in the same manner as the 
thylle* and the gum which incloses the vessels, a separation of 
the inner, uninjured tissues from the wounds.* 
2. The Mechanical System. 
Leaving out of consideration the lower plants, we find that 
all the higher plants which concern us are provided with 
peculiarly formed and characteristically arranged cells, the ex- 
clusive function of which, as Schwendener‘ has shown, is to 
impart to the plant the necessary solidity. While in young and 
still growing organs the collenchyma® (Fig. 109 4, 129 coll) 
represents the mechanical system, in older and matured organs 
this function is assumed by the bast-cells, or stereids° (Figs. 
110, 111). 
The bast-cells, or the specifically mechanical elements of the 
matured plant, form very elongated cells, which are pointed at: 
‘Bachmann, ‘‘ Korkwucherungen auf den Blattern,” Pringsheim’s 
Jahrb., xii., 1880, 
* Sachs, ‘Lehrbuch der Botanik,” iv. (1874), pp. 27, 782. Weiss, 
** Anatomie,” 21, : 
* With regard to the lenticels, see the chapter on the Aérating System: 
*** Das mechanische Prinzip im anatomischen Baue der Monocotylen. 
Leipzig, 1874, z 
*Ambronn, “ Ueber die Entwickelungsgeschichte und die mechani- 
schen Eigenschaften des Collenchyms.” Pringsheim’s Jahrb., xii. E. 
Giltay, ‘‘ Het Collenchym,” Inaugural dissertation, Leyden, 1882. 
* The tissue of bast-cells may be termed stereom. (The pleurenchyma 
of Meyen.) ‘ 
