As to the Mode of Action of the Guard Cells in Dry Air
209
(a)
(b)
Fig. IO.2a,b. Stoma of Sprekelia (Amaryllis) formosissima. a Median transverse and
surface views, b surface view of guard cell with pore closed, and with pore open. (After
Schwendener 1881). Superimposed on b is an indication of the orientation of cell wall
microfibrils
neighboring cells, with a concomitant bending of the ventral wall and opening of the pore. The mechanical basis of opening became more obvious
when Ziegenspeck (1938, 1955) showed that the cellulose microfibrils of the
guard cell walls were oriented in the way superimposed in Fig. 1O.2b,
suggesting that the length of the guard cell is more readily extensible than
the transverse perimeter.
Three other important features of stomatal opening are to be found in
Fig. 10.2b, two of them obvious, the other less so. Although the guard cell
with the wider pore has thickened and bulged at its poles, the central lengthwise axis of the apparatus has not elongated. Secondly, the length of the
pore, as defined by the thickened and ridged upper parts of the ventral