292 THE BIOLOGY OF FLOWERING PLANTS 



The cause of the different types of eccentricity is not yet 

 completely known. Ewart and Masson- Jones (1906) found 

 the formation of red wood in the conifers to be caused by 

 gravity. Recent work by Engler (191 8) seems to show that 

 in the conifers the compression of the cambiaL cells is 

 determining, while in the broad-leaved trees the gravitational 

 stimulus is effective. 



§ 4. Mechanical Features of Leaves 



Leaves are particularly subject to bending. Ordinary 

 broad leaves are in most cases sufficiently supported by their 

 turgor, helped out by the network of interlacing veins. In 

 long sword-shaped and linear leaves, such as those of the 

 grasses, the tendency to double over is much greater. The 

 leaf may be attached round nearly the whole circumference 

 of the stem, as in the sedges, or it may be actually sheathing 

 as in the grasses ; in both cases the tubular form of the basal 

 portion ensures that this region shall be erect. Frequently 

 resupination of the free portion of the leaf-blade is to be 

 seen, that is, twisting on its axis once or more often ; this 

 is a modification of, or approach to, the tubular type of 

 structure. The sclerenchymatous tissue which makes the 

 leaves of so many grasses tough is habitually arranged in 

 girder fashion, the strands running from one surface of the 

 leaf to the other, or bands on the upper and lower surfaces 

 being joined by the vascular bundles and their sheaths. 



The broad expanse of the leaf renders it peculiarly 

 susceptible to tearing. This danger may be lessened if the 

 leaf is more or less divided or is compound, one might say 

 naturally torn, so that the resistance to air currents is 

 diminished. The great leaves of the palms with their 

 feathery divisions afford a good example of this. The 

 epidermal cells of the leaf margin are normally more thickly 

 walled than those of the two surfaces. Along the margin, 

 and in the bays of lobes, where tearing is most liable to 

 occur, strands of sclerenchyma, or marginal veins, may 

 give extra strength, as, for example, in the leaf of the holly. 



