72 EFFECT OF PHYSICAL FACTORS AND PLANT CONDITIONS. 



included with essentially the same water-content as plot 3, but in 

 different stages of growth. In plot 6, the plants were 0.8 dm. 

 high, in 7, 2.0 to 2.5 dm. high, and in plot 3 they had reached a 

 height of 3.5 to 4.5 dm. and were beginning to flower. The dif- 

 ferences found in stomatal movement were insignificant and readily 

 explained by the slight differences in water-content and error of 

 measurement. In series 34 the experiment was repeated. The 

 new growth of the first plot averaged 0.7 dm., the second, 2.0 dm. 

 and in the third plants were just starting to blossom. As the water- 

 content was lower and evaporation higher, 2 hours of mid-day 

 closure was found in the second plot, none in the first, and slight 

 closure in the third. The greater proportion of roots to leaf area in 

 the first plot explains its lack of closure, but the reason for the 

 greater opening in the blossoming plants as compared with the 

 half-grown plants is not so clear. 



The age of the leaf also affects its stomatal movement. This was 

 investigated in alfalfa, barley, corn, Rumex patientia, sweet clover, 

 turnip, potato, and sugar-beet. In all these the stomata are the 

 last of the epidermal cells, and probably of all the cells in the leaf, 

 to be formed. With most of the plants they are formed at nearly 

 the same time, potato being the one conspicuous exception. In 

 consequence, they also begin to function at the same time, a few 

 starting a day or two earlier than the others. At first, opening is 

 slight and very brief, but after a few days it becomes more pro- 

 longed. However, a certain number are badly formed, resulting in 

 mechanically or physiologically defective stomata which never open. 

 About the time that the stomata approach maturity the granular 

 protoplasm of the epidermal cells begins to disappear, and at matu- 

 rity these are rather transparent and clear. The length of life of a 

 mature leaf depends upon the species of the plant and the treat- 

 ment to which it is subjected. Bruising by wind, injury by insects 

 and fungi, exposure to extreme evaporation when the water-supply 

 is deficient, drying out because of stomata wedged open by dust 

 grains and other vicissitudes, often shorten its life. The larger 

 number wither in the natural course of growth, the first visible 

 indication of which is a slight yellowing. But it is usually unsafe 

 to strip the next younger leaf on the stem, since its stomata have 

 begun to function poorly and, before any very evident change of 

 color occurs, they have closed permanently. 



Other conditions in the leaf affect stomatal movement. The 

 relation of the stomata of the two surfaces to the water-supply 

 affects this movement, as shown in connection with sugar-beet. 

 The number of stomata on the leaf and their size is a result of the 

 degree of expansion of the leaf, which in turn was determined in 

 part by weather conditions during development. Thus, a leaf of 

 Malva rotundifolia had 241 stomata per square millimeter and 

 averaging 6.1 M- long, when developed during the hot, dry weather 



