PLANT HABIT AND CONDITION. 71 



and cause closure while they still have a distinctly higher concentra- 

 tion than the adjacent cells. The solution of this problem therefore 

 requires further investigation. 



PLANT HABIT AND CONDITION. 



The growth habit of a plant influences the stomatal movement by 

 introducing differences in the relation of the rest of the plant to 

 the stomata. Thus, the trees investigated did not exhibit mid-day 

 closure, even on days when this was extreme in all the herbs studied. 

 The greater depth to which tree roots penetrate would permit these 

 to draw upon supplies of moisture not available to many other 

 plants. This, however, does not explain why the Lombardy poplar 

 showed no mid-day closure on a day when alfalfa exhibited extended 

 and complete mid-day closure, since the roots of both plants reached 

 the moist soil just above the water-table. As this type of behavior 

 was found in all the trees studied, it is undoubtedly the result of 

 their growth habit and may be attributed to the great amount of 

 water present in the trunk, which acts as a reserve. Dendrograph 

 studies carried on with Pseudotsuga taxifolia and Pinus ponderosa 

 at the Alpine Laboratory show that the water present in the trunks 

 probably moves with great rapidity up to the leaves. 



Stomatal movement in herbaceous plants is subject to great 

 variation as a rule, because the leaf has no reserve of water to draw 

 upon. Hence, when the working margin has disappeared, it is 

 necessary for such a leaf to restrict water-loss and reduce its functions 

 accordingly. The thin-leaved plants naturally have a smaller 

 margin of water than the thicker-leaved ones, and hence show 

 earlier and more extended closure as a rule. With a moderate 

 water-content such leaves may show several periods of day closure 

 under extreme conditions of evaporation. Fleshy-leaved forms, on 

 the other hand, rarely show more than one, and mid-day closure is 

 accompanied by visible wilting in all the cases studied. All of the 

 plants in the group with normally open stomata at night under 

 optimum conditions are rather thick-leaved. As the stomata of 

 alfalfa opened at night after normal day opening only when the 

 water-content was abnormally high, night opening generally is due 

 probably to a greater available supply of water in the thicker leaves. 

 Some of the shrubs showed the same kind of behavior as the trees, 

 but others exhibited some degree of mid-day closure at times, and are 

 intermediate in behavior between trees and herbs. 



The age or degree of maturity, as well as the growth habit of a 

 plant, affects its stomatal movement. Young cabbage plants showed 

 greater and longer continued opening during the day than plants 

 ready to head. Ripening barley showed no movement at the time 

 when plants started later and not yet headed out showed 4 hours 

 opening during the morning. In series 32, two other plots were 



