THE AXGIOSPERMAE : LEAVES 1017 



This account of the reactions, though it is consistent with ohsen-ations 

 and with experimental results, so far as they go, must be regarded as still 

 hypothetical, since it has not been definitely proved that photosynthesis 

 occurs in stomatal plastids, nor has the presence of amylase in the guard cells 

 been demonstrated. 



Scarth considers that the opening and closing movements are too rapid 

 to be accounted for by enzymatic reactions, and considers that the changes 

 of acidity, which do take place, affect the imbibitional swelling of the cell 

 colloids and that the changes of turgidity are, at least in part, due to changes 

 of water content due to colloidal imbibition. As the water drawn in to the 

 guard cells must come from the leaf tissues, it follows that the possibility of 

 movement will be afi^ected by the water content of the leaf as a whole. A 

 decrease of mesophyll water content, due to wilting, will increase the osmotic 

 potential of the mesophyll cells and when this has passed a certain value it 

 will no longer be possible for the guard cells to withdraw water from them 

 and the stomata will remain permanently shut, thus minimizing the further 

 loss of water from the leaf. 



Finallv it may be said that recent observations have detected a rhythmical 

 pulsation in guard cells, with a fifteen-minute period, which is even maintained 

 for twelve hours in continuous light. To what this is due is not known, though 

 a short-period opening and closing can be produced, experimentally, by 

 variations of tissue water content, under extreme environmental conditions. 



The classic researches of Loftfield have shown that stomata do not all 

 react similarly to light and that they generally conform to one of three types. 



Type I. — Typical of Grasses, including the cereals. During the day the 

 stomata may open and close rapidly, but they are only open for an hour or 

 two altogether and only under the most favourable conditions. There is no 

 opening at night. 



Type 2. — Includes most thin leaved mesophytes.^ Usually the stomata 

 open all day and close all night. Under less favourable conditions they may 

 close at midday, and under very unfavourable conditions, especially heat and 

 drought, this may extend to the whole day (Fig. 1005). Night opening is the 

 obverse of day opening, so that under very dry conditions stomata may be closed 

 all day and open all night, depending on the water content of the tissues. 



Type 3.^The " Potato Tvpe " (Fig. 1006). Under favourable conditions 

 the stomata are open more or less all day and all night, closure being caused by 

 increased evaporation rather than by darkness. With diminished water content 

 they become more responsive to light and then they tend to conform to Type 2. 



The stomata of many water plants have no powder of movement and remain 

 open even when the leaves are wilted. Immobile and enlarged stomata may 

 also function as water stomata through which water is excreted under very 

 moist conditions. They occur principally at the margins of leaves, as in 

 Fuchsia, Tropaeolum and Primula, or at the leaf tips, as in the primary leaves 

 of many Grasses. These water stomata form one of the types of water-secreting 

 structures, or hydathodes (see p. 472). 



1 Plants living under normal environmental conditions. 

 33 A 



