170 



BOTANY 



represents a transverse section through a hand-like leaf of Phormium 

 tenctx, the New Zealand flax, which may reach a length of two metres; 

 it illustrates how such a leaf is strengthened by sclerenchymatous 



.d 



Fig. 169. — Disposition of mechanical tissue to secure rigidity. Transverse sections, 1, of a young 

 shoot of Sambucus ; 2, of the floral shoot of Eryngium ; 3, of a leaf of Phomiium tenax ; 1, of a 

 root ; e, collenchyma ; s, sclerenchyma functioning as mechanical tissue (deeply shaded) ; g, 

 green, and w t colourless leaf parenchyma ; h, hypodenua. 



plates and strands. The mechanical elements of this leaf afford the 

 strongest English ships' cables. 



Where, however, pressure must be guarded against (as in Plum- 

 stones, and in Hazel and Walnuts), the 

 mechanical resistance is maintained by 

 an arching mass of sclerotic cells, which, 

 like sclerenchymatous fibres, are often 

 further strengthened by deposits of 

 mineral matter. 



Stems of trees which have to support 

 heavy and frequently large crowns, must, 

 like pillars, be constructed to withstand 

 pressure and bending. 



All such heavily thickened, inflexible 

 rigidity by folding and roiling, in skeletal elements have lost their capacity 



the outspread organ A, to withstand P ., -, . .-, P -, 



a pressure acting in the direction of for growth, and cannot, therefore, be 

 the arrow, only the thickness d is utilised in those parts of plants which 



available, but in the folded organ B are in an act j ve l y growing State. In 

 the thickness D , and in the rolled . . ■/ c ? . . . . 



organ the thickness d". such cases where greater rigidity is re- 



quired than can be maintained by cell 

 turgidity and tissue tension, it is secured by the development of 

 collenchyma (p. 78). This tissue, according to Ambronn's re- 

 searches, in addition to its extreme resistance to tearing, possesses the 

 power of elongating under the influence of the force of growth. The 



Fig. 170. — Diagrammatic transverse 

 sections, to show method of securing 



