250 



LECTURE XV. 



the thick ridges a and 6, which are fastened at the two ends of the guard-cells (of. 

 Fig. 203, 6)f must become curved by this ; and by means of their elasticity they bring 

 it about, that, as soon as the turgescence is diminished, the guard-cell becomes com- 

 pressed and straight, and that the walls are driven outwards as in Fig. 203. These 

 are only the most important and obvious points in the mechanics of the opening and 

 closing of the stomata. So far as concerns the transverse section of a guard-cell, the 

 matter may be briefly put thus : the guard-cell, as turgescence increases, tends to 

 assume a more symmetrical form, and, as the turgescence diminishes and the 

 aperture closes, a less symmetrical form, as is shown at once by Fig. 203 '. 



Although the desired certainty does not yet exist with respect to the external 

 conditions under which the stomata open and close, so much is at any rate certain, 

 that the apertures open in sunshine, and under strong illumination generally, when the 

 guard-cells increase in turgescence; and that they close in shade and darkness, when the 

 turgescence of the guard-cells diminishes. In the first place, it is clear that the altera- 

 tions in turgescence must result from the entrance and exit of water ; and that in this 

 process it is the epidermal cells bordering upon the guard-cells which- give up or again 



absorb the water. In this connection the thin 

 lamella e, e (Fig. 203) is important, in so far 

 that it facilitates the passage of the water. 

 The question is now, how does the light bring 

 about an increase of the turgescence of the 

 guard-cells ; and why does this diminish with 

 declining illumination ? With regard to these 

 points, Schwendener, from whom the preced- 

 ing description is in the main borrowed, has 

 expressed no opinion. It appears to me, 

 however, that the fact which I have repeatedly 

 brought forward during the last twenty years 

 — that the guard-cells of all foliage leaves 

 contain chlorophyll-grains and the products 

 of their assimilation, which are usually wanting in the surrounding epidermal cells — 

 can now be explained. The chlorophyll>grains in the guard-cells will, as the 

 illumination increases, produce carbo-hydrates by assimilation; these are at least 

 partly soluble, and act endosmotically, and thus bring about an inflow of water 

 from the neighbouring epidermal cells through the thin boundary wall. Or at 

 least so much may be assumed with certainty, that by the presence of the chloro- 

 phyll in the guard-cells generally, a supply of organic substance is ensured in 

 them, which according to circumstances may cause a diffusion current. It will 

 require further researches to decide whether this is really sufficient. Besides this, 

 however, a direct influence of light upon the molecular condition of the protoplasm 

 may come into consideration ; so, indeed, that we may here have to do with a phe- 

 nomenon of irritability in the narrower sense of the word. We may imagine that, 

 as the intensity of the light increases, the protoplasmic utricle clothing the cell-wall 

 becomes more resistant, opposes the hydrostatic pressure of the cell-sap more strongly, 



Fig. 203 d. — Stoma seen from above (externally). The 

 upper half of the figure shows the form of a guard-cell 

 wben the stoma is closed ; the lower half that when 

 open. 



* See note i, p. 248. 



