MECHANICAL CONSTRUCTION OF PLANT-BODY 157 



corresponding involution of the surface between the girders, while the lattei are 

 specially deep. The upper leaf-surface is thus marked by parallel grooves, 

 the aqueous cells at the bottom of each furrow being large and thin-walled. 

 The lower side of the leaf is sclerotic, so as to maintain its outline, while the 

 upper is liable to shrink with drought. Such shrinkage draws the margins 

 together. The sloping faces of the grooves, which bear the stomata meet, and 

 transpiration is checked. Access of water, on the other hand, swells the 

 aqueous cells, and the leaf flattens out again. 



The Monocotyledon leaf, with its parallel venation is usually secure 

 from marginal tearing. A prominent exception is seen in the Banana, 

 where the huge leal seldom appears 

 perfect in plants grown in the open. 

 It has a midrib from which the veins 

 run out parallel towards the margin, on 

 the plan of a feather. These leaves 

 are readily slit to ribbons by the wind, 

 from the margin inwards, since there 

 is no sufficient marginal protection. 

 In most Palms a similar subdivision 

 of the leaf is carried out during de- 

 velopment, by the plants themselves. 

 Certain tissue-tracts dry up, cutting 

 the blade into segments, and giving 

 the appearance when mature of a 

 true pinnation. For smaller leaves 

 tearing from the margin is still a risk, 

 and there are structural arrangements 

 which prevent it. The commonest is 

 the curving of the veins into intra- 

 marginal arches (Fig. 116). Several series of these of successively 

 smaller size towards the margin, effectually check tearing. The 

 most dangerous spots are naturally the indentations in toothed or 

 deeply cut leaves. These are often protected by small " gussets " 

 of indurated tissue at the base of each sinus. Good examples are seen 

 in Ferns, as in the genus Asplenium (Fig. 117) ; or a special strongly 

 arched vein may run across the point of deepest indentation ; while 

 not uncommonly a patch of sclerotic cells may be fused with it at 

 the point of danger (Fig. 118). This is often continued along the 

 margin as a sclerotic band, which serves, like the hem of a hand- 

 kerchief, to prevent a marginal tearing. Similar hems are found in 

 many ordinary leaves, but conspicuously in xerophilous plants. 

 Good examples are seen in the Date Palm, and in the Gum Trees. The 



FIG. 117. 



Part of leaf of Asplenium horridum, 

 showing "gussets" dotted, at the base of 

 indentations. (Slightly enlarged.) F. O. B. 





