344 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 



twenty-nine ridges can be counted on the leaf of the above-mentioned Lasiagrostis, 

 a plant widely distributed in the valleys of the Western and Southern Alps, where 

 it clothes the sunny slopes in thick masses. When the leaf folds up, the twenty- 

 eight grooves between the ridges, on whose sides are the stomata, become narrowed, 

 and the entire leaf assumes a tubular form, so that transpiration is almost com- 

 pletely suspended. In Stipa capillata, which is very abundant on clay steppes, 

 the same thing occurs (see fig. 86 ^ ). In both grasses the closure of the grooves on 

 whose sides are the stomata, is completed by short stiff hairs on the summit of the 

 ridges, which interlock when the ridges approach one another, and so block up 

 access to the grooves (fig. 86 ^). It would take us much too far to describe the 

 numerous other modifications which are to be met with in the structure of hinged 

 grass -leaves. The examples given suffice to make it evident that the danger 

 of over-transpiration is avoided by the folding of the leaf, and that amongst 

 the grasses very many arrangements obtain in order, sometimes, to expose those 

 green parts of the leaf whose epidermis is supplied with stomata to the rays of the 

 sun, and at other times to withdraw them, according to the humidity of the soil 

 and of the surrounding air, thus suitably regulating transpiration to the existing 

 circumstances. 



The mechanism by which grass-leaves open and close may be explained in two 

 ways — either the process is due to hygroscopic changes, as in the opening and clos- 

 ing of the " Rose of Jericho ", or to alterations in the turgidity of certain groups of 

 cells, as in the mimosas. If the former alone were the case, a dry, dead grass-leaf 

 should be still capable of opening and closing in accordance with its damp or dry 

 condition; but a leaf of any of these when cut off and dried no longer opens, even 

 after being moistened for a considerable time, and therefore the first explanation 

 cannot be accepted, at any rate for most of the grasses. Apparently, the mechanism 

 consists of alterations in the turgescence of those groups of cells situated in the 

 angle of the grooves. Since the floor of the grooves was frequently found to con- 

 sist of peculiar thin-walled cells destitute of chlorophyll, and filled with colourless 

 watery sap, it was concluded that the opening and closing of the grass-leaves was 

 due to the change in turgidity of these cells. However, this was going too far. 

 These cells in most instances, for example, in Festuca punctoria (see fig. 88 -), would 

 be much too delicate to effect, unaided, the closure of the leaf by their loss of 

 turgidity, or to open it by their increasing turgescence. In many grasses these 

 cells are completely wanting {e.g. in Festuca alpestris and Stipa capillata, fig. 86). 

 Moreover, it is observed that the opening and closing of the leaf is still carried on 

 when the thin-walled cells at the bottom of the grooves are destroyed, artificially, by 

 puncturing with fine needles. The cause of the movement must therefore be looked 

 for in the alteration of turgescence of other cells below the grooves. When a 

 mantle of several layers of thick-walled cells is present on the under side of the 

 leaf, their walls are seen to swell up simultaneously with the alterations of tur- 

 gescence of the parenchymatous cells. Of course the inner cell-layers of the mantle 

 must be capable of swelling up to a greater extent than the outer, and this has 



