92 



BOTANY OF THE LIVING PLANT 



internal surface, since the convex wall presents a larger area, it will stretch 

 more than the concave wall ; and this would in itself produce a greater curva- 

 ture, even if the walls were all of the same thickness. But they are not. 

 The ridges of thickened cell-wall on the outer and inner faces of each guard-cell 

 make the average thickness of the wall greater on the side next the pore. 

 This will accentuate the curvature of the cells whenever their osmotic turgor 

 rises. A further effect of the turgor will be to make the section of the cell- 

 cavity approach as nearly as possible to the circular. This will result in 

 withdrawing the projecting surfaces where the cells meet when closed. All 

 these factors act together in opening the stoma. The increase of turgor is 

 secured by the action of the chlorophyll whenever the guard-cells are exposed 

 to sufficiently intense light. The consequence is that in daytime the stomata 

 are habitually open. But in darkness, since the osmotic substances may be 

 quickly drafted away to other cells, the turgor falls, and the stoma closes. 



Under conditions of night the transpiration falls off. The roots 

 still continuing to absorb, their action becomes the more prominent 



factor. Water may thus at 

 night gorge the tissues of the 

 plant, and is often seen to 

 exude at the tips and margins 

 of the leaves. Here in many 

 plants water-glands exist, 

 which by secreting watei pro- 

 vide as safety-valves against 

 the risk of v/ater-logging of the 

 intercellular spaces (Fig. 69). 

 The water thus extruded at 

 night from the leaves of 

 Grasses is often mistaken for 

 dew. A somewhat similar 

 condition is seen in spring in woody trees before the leaves are fully 

 developed. At this time if the stems be cut, water will exude in 

 considerable quantity, a phenomenon known as "bleeding." These 

 are consequences of the continued activity of Root when transpiration 

 is not active. 



The Transpiration-Stream providing the means of transit of salts 

 in solution, and the water evaporating away, these salts will be con- 

 centrated at the place where the evaporation is active : that is, in 

 the cells of the mesophyll of the leaf. Their cell-walls saturated with 

 water abut directly upon the intercellular spaces. The water-vapour 

 given off from them passes out, under control, through the pores of the 

 stoma. Now the cells of the mesophyll are those which are most 

 active in Photo-synthesis, for which certain of the salts are required. 



Fig. 69. 

 Exudation of water from the margin of the leaf of 

 Tropaeolum, through water-stomata which are perma- 

 nently open. (After Strasburger.) 





