MECHANICAL CONSTRUCTION OF PLANT- BODY 153 



Similar principles hold also for the stems of Monocotyledons ; but the 

 circumstance that their closed vascular strands are usually scattered over the 

 transverse section tends to complicate their scheme of construction (p. 44). 

 On the other hand the absence of cambial thickening makes its recognition in the 

 mature stem easier. Commonly the sclerotic tissue accompanies each vascular 

 strand, as bands following its course, or even as a sheath encircling it (Fig. 32). 

 Such an arrangement is in itself comparable with a girder, and occasionally, 

 where there is a ring of vascular strands thus invested, the construction is 

 simply a circle of sub-epidermal girders, as in Scirpus (Eleocharis) caespitosus 

 (Fig. in). Sometimes the sclerotic bands may be quite separate from the 



FIG. 112. 



Transverse section of the flowering shaft of Molinia coerulea. Centrally is a large 

 cavity. Thin-walled tissue is left clear : sclerotic tissue is dotted, vascular strands 

 cross-hatched. The peripheral vascular strands are embedded in a continuous ring 

 of mechanical tissue. ( x 40.) 



vascular strands, though usually opposite to the strongest of them, as in 

 Schoenus nigricans (compare Fig. 29, p. 44). These simple arrangements are 

 very like those seen in the Dicotyledons. But in most Monocotyledons the 

 vascular strands are scattered over the transverse section, and this introduces a 

 good deal of variety of detail. Where the vascular strands are thus scattered, 

 and the mechanical construction more complicated, the sclerotic bands may be 

 fused together laterally in various ways. A continuous sclerotic ring may thus 

 be formed, and the vascular strands be embedded in it, with or without 

 flanges projecting from it (Fig. 112). Such structure developed on a larger 

 scale, and with more general fusion of the sclerotic tissues, leads to the con- 

 dition seen in the large Bamboos and Palms. 



The case of Molinia coerulea proves how effective this mechanical strengthen- 

 ing actually is. The structure of the haulm is shown in transverse section 



