190 



BOTANY OF THE LIVING PLANT 



marginal stiffening becomes actually aggressive in spinous leaves, such 



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



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



A 



Fig. 125. 



" 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. 



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

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

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

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

 adaptability 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. In 

 cordage, in order to resist such tension, 

 the fibres are twisted together so as to be 

 grouped in as small a transverse area as 

 possible. This method secures the even 

 distribution of the stress over them all. 

 A similar condensation of the mechanical 

 tissues, but without the twisting, is usual 

 in roots. Their stele is small compared 

 with the whole transverse section, and it is 

 frequently pithless. But often the xylem 

 stops short of the centre, which is then 

 occupied by sclerotic tissue. This links 

 up the xylem, so as to form with it a 

 resistant central cord (compare Fig. 58, of Acorus). In larger roots 

 a pith may be present, surrounded by a dense ring of mechanically 

 effective tissue composed in the same way (Fig. 126). But still the 



Fig. 126. 



Transverse section of root of Rus- 

 ius showing large proportion of cortex 

 to the contracted and pithed stele. 

 :z.) F.O.B. 



