Aeration in Leaves 269 



an air passage. If the petiole of such a leaf is arranged as above indi- 

 cated the air may not issue from the cut end of the petiole for a very- 

 long time, or not at all under slight vacuum. At times as much as 30 

 minutes elapse under slight vacuum before the air passage begins. 

 When, however, it has once begun it continues with great rapidity until 

 the expelling force is removed. When the passage of air does not begin 

 at once the suction should be increased by lengthening the mercury or 

 water column till the flow of bubbles commences. The force employed 

 may then be decreased if desired to the minimum amount that will 

 sustain a continuous stream of air bubbles. The stream of air bubbles 

 will cease as soon as the leaf is immersed and will commence again as 

 soon as it is restored to the air. This proves that the failure of air 

 to pass at first under slight vacuum was not caused by the stomata 

 being blocked with water, but was due to other causes. Large quantities 

 of air issued even under the small vacuum. The leaves tested were of 

 medium size, being about 12 cm. wide and 14 cm. long, and were vigorous 

 in every respect. 



Myriophyllum proserpinacoidea allows air to pass with ease but not 

 quite so easily as Nymphaea odoratu. For example, a vacuum of 16 mm. 

 of mercury was required to cause the air to flow through this plant from 

 the leaves. The volume of air passed through was much less than for 

 A^ odorata. For example, it required five minutes for 1 cc. of air to 

 emerge from the stem. The combined amount of surface of many leaves 

 of M. proserpinacoides is much smaller than a single one of N. odorata 

 so that the difference per unit area is not so great as might be expected. 



The genus Rumex offers a very interesting land type of this function 

 in contradistinction to the two foregoing aquatic types. The species 

 experimented with was R. obtusifolius. Leaves 10 cm. wide and 16 cm. 

 long were used. In these leaves a vacuum of only 28 mm. of mercury 

 was required to cause the air to flow through the stomata and out of 

 the petiole. Quantitatively the amount of air passed through the leaf 

 was much less than in either of the first two plants discussed. At a 

 minimum vacuum of 28 mm. of mercury only 1 cc. of air passed through 

 the petiole in 17 minutes. As in the case of the first twQ plants, this 

 flow instantly ceased when the leaf was submerged in water but began 

 again when restored to the air. This, however, did not occur quite as 

 quickly in the case of R. obtusifolius. The stomata of the leaves of 

 R. obtusifolius are large. If one knows the size of the stomata and the 

 volume of the air which issues, the number of stomata per unit area may 

 therefore be easily calculated. If the leaves of these plants are attached 

 directly through rubber stoppers or in tubes the amount of vacuum neces- 

 sary to bring about the desired result is so small that the union may 

 easily be made air tight by the use of plastilina. While in most plants 

 the interchange of air is effected with much greater difficulty, the quanti- 

 tative estimations here given for the three plants above mentioned show 

 clearly the decided capabilities of certain plants in this respect. Rumex 

 ubtm^ifolius and Myriophyllum. proserpinacoides can be used as excellent 

 demonstration material before an audience. When R. obtusifolius was 

 connected with a 1200 cc. flask on which a vacuum of 20 cm. of mercury 



