156 



BOTANY OF THE LIVING PLANT 



veins are then cat at right-angles, and ni simple cases eaeli may present 

 an appearance as in Fig. io6, p. 148, of Cy penis. 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 

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

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

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



Fig. 116. 



Photograph of the skeleton of a Dicotyledon leaf, showing retieulatiou with 



successive intra marginal arches. (Natural size.) 



construction. 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- 

 storage (Fig. 113). 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- 

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

 girder-principle may be recognised as underlj'ing sueh deviations. 



The most interesting variants arc those seen in xerophilous grass-leaves, 

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

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

 114 115.) Their mechanism shows a reduction of the mesophyll with a 



