MECHANICS LEAF-MOVEMENTS 115 



section (cf. p. 84), but tend to be more densely crowded near the 

 outside (cf. Fig. 39, p. 84). The bundles are thus again common 

 to stem and leaf. 



The customary disposition of the mechanical elements in 

 petiole and midrib in the form of an arc admits of considerable 

 flexibility under the influence of wind, etc., but at the same time 

 is well suited to withstand the vertical strain caused by the 

 weight of the leaf -blade. In peltate leaves (e.g. Marsh Penny- 

 wort, Hydrocotyle ; Garden Nasturtium, Tropceolum, etc.), where 

 the strain is equally distributed, the petiole, however, exhibits 

 a corresponding radial construction, as in stems. The broadly 

 expanded lamina of the ordinary leaf is exposed to risk of injury 

 under the tearing action of wind and hail, but this is to a large 

 extent obviated by the strengthening network formed by the 

 veins, and especially by the occurrence of marginal mechanical 

 elements. The latter either form arched connections running 

 parallel to the leaf margin, between the finer vascular strands 

 (e.g. Red Currant), or bundles of fibres (e.g. Iris) occupying the 

 same position. Moreover, the epidermal cells at the edge of the 

 leaf are usually especially thickened and have a pronounced 

 cuticle. 



In leaves which normally exhibit sleep-movements 1 (e.g. False 

 Acacia, Robinia pseudacacia ; Runner Bean, Phaseolus multi- 

 florus ; Sensitive Plant, Mimosa pudica], and in which the 

 petiole takes up different positions in darkness and in light, the 

 swollen leaf -base, or pulvinus (Fig. 56, p.), is the region of move- 

 ment. This is facilitated by a flattening of the vascular 

 tissue in a plane at right angles to the direction of curvature 

 (Fig. 56, Pu.), the necessary rigidity being attained by a greater 

 development of the cortex, which consists of large, turgid, thin- 

 walled parenchyma-cells. The fall of the leaflet at dusk is 

 accompanied by a decrease in the turgidity of the cortical cells 

 on the lower side of the pulvinus, whilst at the same time some 

 of their watery sap escapes into the intercellular spaces. This 

 is due to a change in the permeability of the protoplasm and, as 

 a result of the infiltration of the intercellular spaces, the whole 

 pulvinus acquires a more transparent, deeper green appearance. 

 At dawn the cells of the pulvinus once more become turgid and 

 1 Cf. F. & S. p. 221. 



