1 5 8 



ROTANY OF THE LIVING PLANT 



marginal stiffening becomes actually aggressive in spinous leaves, such 

 as the Holly, Barberry, and Gorse. A particular instance of a like 

 sclerotic development that serves a peaceful end is seen in the Sand 

 Sedge. It burrows with its creeping rhizome through the sand. The 



FIG. 118. 



" Gussets " at margins of the indentations of leaves (A) of the Elm, (B) of the 

 Sycamore, showing their relation to the vascular network, and to the mechanically 

 strengthened margin. ( x 14.) F. O. B. 



apical bud has its successive scale-leaves developed to a point, tippec 

 with hard sclerenchyma, by means of which it passes through soft 

 objects like a brad-awl. These are a few examples of the mechanical 

 adaptibility of the leaves of Flowering Plants. 



(c) THE ROPE-REQUIREMENT. 



Those parts of the plant, such as roots or rhizomes, which hold it 

 upright in the soil against the impact of winds are subject to longitu- 

 dinal tension, as on a rope or string. Ir 

 cordage, in order to resist such tensioi 

 the fibres are twisted together so as to be 

 grouped in as small a transverse -area 

 possible. This method secures the ever 

 distribution of the strain over them all 

 A similar condensation of the mechanic; 

 tissues, but without the twisting, is usual 

 in roots. Their stele is small compare 

 with the whole transverse section, and it is 

 frequently pithless. But often the xyler 

 stops short of the centre, which is ther 

 occupied by sclerotic tissue. This links 

 up the xylem, so as to form with it 

 resistant central cord (compare Fig 73, of Acorus). In larger root 

 a pith may be present, surrounded by a. dense ring of mechanical!} 

 effective tissue composed in the same way (Fig. 119). But still the 



FIG. 119. 



Transverse section of root of Rus- 

 cus showing large proportion of cortex 

 to the contracted and pithed stele, 

 (xu.) F.O.B. 



