158 



BOTANY 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. ii8. 

 " Gussets " at margins of the indentations of leaves (A) of tfie Elm, (5) of tfie 

 Sycamore, stiowing their relation to the vascular network, and to the niechanicaUy 

 strengthened margin. ( ^' 14.) F. O. P.. 



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 

 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. 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 strain 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 

 Pio. 119. frequently pithless. But often the xylem 



Transverse section of root of itiis- stops short of the Centre, which is then 



c».^ showing large proportion of cortex 



to the contracted and pithed stele. OCCUpicd bv SclcrOtic tisSUC. This links 



( / r2.) F. O.B. "^ -^ 



up the xylem, so as to form with it a 

 resistant central cord (compare Fig y^, of Acorns). In larger roots 

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

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



