ROLLED LEAVES 



163 



B 



the groove occupying a small area in the lower angle and sur- 

 rounded by the very lacunar mesophyll. The rolled leaves of the 

 Crowberry (Empetrum) have a similar organisation, except that 

 the stomatal chamber is much larger (Fig. 84, A). 



More extreme types of this kind are found in Grasses, where, 

 moreover, the leaf is often capable of rolling and unrolling in 

 response to changes in humidity. An excellent example is 

 afforded by the leaves of the Marram Grass (Psamma arenaria), 

 which clothes many young dunes in great profusion. A section 

 across the rolled-up leaf is more or less circular in outline 

 (Fig. 85, A), and is bounded on the outer 

 (i.e. under) surface by an epidermis pro- 

 vided with a thick cuticle (Cu.) and devoid 

 of stomata ; within are several layers of 

 rounded thick-walled cells (Fig. 85, B, m.). 

 The inner (i.e. upper) surface has a corru- 

 gated appearance, being produced into a 

 number of longitudinal ridges, each of 

 which is traversed by a vascular bundle 

 (V.b.). The intervening grooves are 

 flanked by assimilatory tissue consisting 

 of more or less rounded cells (A.t.). The 

 scattered stomata (5.) correspond in posi- 

 tion with this tissue and are consequently 

 confined to the furrows, where additional 

 protection is furnished by numerous stiff 

 interlocking hairs (Fig. 85, A). 



The epidermal cells at the base of each groove are exceptionally 

 large (hinge-cells, Fig. 85, h. and h.c.) and, being relatively thin- 

 walled, are the first to lose water and shrink when transpiration 

 is excessive. As a result the inner surface of the leaf contracts 

 in width (i.e. transversely), so that the flat edges formed by 

 the marginal ridges are brought together and the leaf as a whole 

 becomes tubular. When the water-supply is plentiful, the 

 reverse action takes place. 



The preceding paragraphs will have indicated the manifold 

 devices that are employed to restrict transpiration from the 

 stomata, and such are particularly characteristic of plants of 

 dry situations. The efficacy of the sunken stoma and of pro- 



FIG. 84. Diagrams of 

 transverse sections 

 of the leaves of the 

 Crowberry (Em- 

 petrum nigrum) (A) 

 and Heather (Calluna 

 vulgaris) (B). St., 

 stomatal chamber. 



