

BOTANY OF THE LIVING PLANT 



nation makes the transverse section appear simpler. All the main 

 ins arc then cu1 at right-angles, and in simple cases each may present 

 an appearance as in Fig. 113, P- 181, ° f Cyperus. Sclerotic strands 

 follow the veins on either side, forming with each a girder, as above 

 explained. Since these girders run parallel, and are held in place by 

 the firm upper and lower epidermis, the construction of the whole is 

 on the same principle as that of a lattice-girder railway-bridge, in 

 winch also a high degree of rigidity is required, together with economy 

 of material. In some, as in the old Charing Cross railway-bridge, the 



Fig. 123. 



Photograph of the skeleton of a Dicotyledon leaf, showing reticulation witn 

 successive intra-marginal arches. (Natural size.) 



tracks run between the girders, that space being left vacant in the 

 nstruction. In many leaves of Monocotyledons the corresponding 

 space between the girders is occupied by mechanically ineffective 

 mesophyll, while in some there are large thin-walled cells for water- 

 ge (Fig. 120). It thus appears that the requisite stiffness of 

 Monocotyledon-leaves is gained by means very similar to those 

 employed by engineers to obtain like results in bridges. But irregulari- 

 ties are frequent, especially in thick leaves. The girders may be in- 

 mplete, or the sclerotic bands may be fused laterally. But still the 

 girder-principle may be recognised as underlying such deviations. 



I he most interesting variants are those seen in xerophilous grass-leaves, 



which curl automatically so as to check transpiration. Native examples are 



Sheep's Fescue, the Marram, Lyme, and Tussock Grasses (Figs. 



120-122). Their mechanism shows a reduction of the mesophyll with a 



