FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 



345 



actually been shown to be the case in some species. Moreover, although the thin- 

 walled cells at the bottom of the furrows are not consiaered strong enough to bring 

 about the opening and closing by changes in their turgidity alone, it is by no means 

 asserted that they have no other part to play. When they are constructed as in 

 the leaves of the moor-grasses and in the fescue-grass of the Taurus (Festuca 

 punctoria, figs. 85 and 88), they certainly are not without a purpose. Their advan- 

 tage to the plant lies in the fact that they can be much compressed without harm 

 by the closure of the leaf, whereby the neighbouring parenchymatous cells are pro- 



■^^^ 



Fig. 88.— Folding of Grass-leaves. 



^ Vertical section through an open leaf of Festuca punctoria, of the Taurus. * Vertical section through a closed leaf ; x 40. 

 8 Vertical section through a portion of the open leaf ; x280. 



tected from injury; also that by means of these cells, which are filled with watery 

 «ap, carbonic acid from the atmosphere is conducted to the underlying green tissue; 

 and lastly, that in case of necessity, water can be absorbed from the air. They re- 

 mind one strongly of the thin-walled groups of cells of foliage-leaves used for the 

 direct absorption of moisture, and possibly they can function in this way. If, 

 in places where these grasses grow naturally, a slight shower of rain falls after a 

 long period of drought, or if dew falls during clear nights, little or none of the 

 water reaches the roots, since it is retained by leaves overspreading the soil. But 

 the water easily runs into the furrows of the folded leaves of grass, and since the 

 large thin-walled cells at the bottom of the grooves can be wetted, they offer to the 

 water which can pass through them the shortest path to the green cells in the 

 interior of the leaf. 



