PROTECTION OF LEAF-MARGINS 



101 



frequent, method of protecting the leaf-margin consists in an increase in 

 the thickness of the outer epidermal walls ; the marginal cells often 

 differ very noticeably from those of the leaf-surface in this respect. 

 Frequently subepidermal layers, consisting either of more or less 

 thick-walled collenchyma (Aroideae), or of well -differentiated bast- 

 fibres, are also employed to strengthen the leaf-margin. The cross- 

 sectional outline of these subepidermal collenchymatous or fibrous 



C 

 Fro. 67 



Mechanical devices for strengthening the margins of leaves. A-D. Cross-sections 

 of various leaf -margins. A. Theophrasta Jussieui ; b, a subepidermal marginal 

 fibrous strand of irregular outline. B. T. imperial-is. The marginal fibrous strand, 6, 

 m which two mestome-bundles are embedded, is separated from the epidermis by a 

 few layers of parenchyma. C. Eucalyptus sp. ; c, semi-lunar subepidermal plate of col- 

 lenchyma. D. Plioenix ductylifera (pinna) ; b, fibrous strands beneath the upper and 

 lower angles of the margin ; :, remains of the strip of tissue by means of which the 

 pinna was formerly connected to its neighbour. '. Sanguisorba carnea. Tangential 

 section through an indentation of the leaf -margin. Tearing is prevented by a sickle- 

 shaped mass of collenchyma, which is further supported by a tangential mestome- 

 anastomosis. (All that is shown of this anastomosis is a single row of tracheides 

 together with part of the parenchymatous bundle-sheath.) F. Part of the leaf-margin 

 of Kibes rubrum in surface-view ; each indentation is strengthened by a mestome- 

 anastomosis ((/), which all but intersects its base. 



groups varies greatly according to circumstances. Where the leaf has 

 a sharp edge, the subepidermal fibrous strand is often sagittate in 

 cross-section (e.g. in Iris, Eryngium, etc.). If the edge is rounded, the 

 marginal strip of stereome usually has a sickle-shaped or semi-lunar 

 cross-section (Eucalyptus sp., Fig. 67 c); or it may be broken up into 

 several small strands, which are irregular in shape and unequal in 

 size. In the Date Palm the pinnae appear to be cut off short along 



