286 THE BIOLOGY OF FLOWERING PLANTS 



wood from which the other types have been evolved, on 

 the one hand vessels, the perfect conducting elements, and 

 on the other fibres, the most efficient mechanical cells. 

 (<:) Fibres are very narrow elongated cells with sharp, 

 tapering, sometimes split, points, which are, as it were, 

 spliced into each other by sliding growth in the course of 

 development. Their walls are so much thickened that only 

 a narrow lumen is left, and the pits are minute. They are 

 purely mechanical in function. Of the same nature are the 

 bast fibres associated with the phloem, as in the lime, and 

 the sclerenchyma fibres which occur in strands and belts 

 in the cortex and ground tissue, and are specially prominent 

 and important in monocotyledonous stems and leaves. 

 (d) The short, thick sclerotic cells, or stone cells, have much 

 thickened lignified walls with small pits ; they occur often 

 isolated in leaves, and form the stony tissues of fruit and 

 seed walls. A well-known example of stone cells is 

 afforded by the gritty flesh of the pear. 



The woody fibres are extremely strong. Haberlandt 

 quotes figures for the sclerenchyma of various plants ; we 

 may give one typical example, that of Hyacinthus orientalis^ 

 the fibres of which support a weight of 12 kilos, per square 

 millimetre of cross section without losing their elasticity, 

 and of 16 kilos, at the breaking point. The corresponding 

 figures for wrought iron are 13 kilos, and 40*9 kilos. 



§2. Mechanical Features of the Root System 



The land plant is fixed in the soil by its roots ; as the 

 stem moves in the wind the roots on one side are brought 

 under a strain. In unsymmetrical plants a unilateral 

 strain may be more or less permanent. In any case a 

 longitudinal strain is the chief force which roots must resist. 

 For a given material the resistance offered to such a strain 

 is directly proportional to the area of the cross section ; the 

 arrangement of the material does not matter. In fact we 

 find in roots a great uniformity of arrangement. In the 

 young root the wood bundles are grouped near the centre 



