1910.] 



Vegetable Assimilation and Respiration. 



449 



the most satisfactory explanation of this : thus even very bright diffuse Ivjht 

 appears to he insnj/iricnt to induce the stomata o/H. annuus to open to tJieir full 

 extent. It is suggested that this depends, not on the photosynthetic value of 

 the radiation, which may be great, but on some other quality, such as its 

 relative poverty in heat rays. 



3. Compared with Hrlianthus, Catalpa hignonioides assimilates at much 

 lower rates, a fact wliich is correlated with the absence of stomata from the 

 upper surface of its leaves. Under conditions which enable leaves of 

 Helianthvs to increase in dry weight by 17 milligrammes per square decimetre 

 per hour, Catalpa leaves show an increase of 5 or 6 milligrammes per hour. 



Experiments with Catalpa, unlike those with Helianthus, have not given 

 results far above those which Brown and Escombe obtained with the same 

 plant. Brown and Escombe's more normal values with this plant indicate 

 that its leaves stand exposure in a glass case better, so that they were able 

 to carry out an experiment in intermittent sunlight. In my most successful 

 experiment, however, the average result for 12 leaves exceeds their highest 

 result ; and if differences in the stomatal apertures of different leaves are 

 taken into account, individual leaves must have increased in weight still 

 more rapidly. 



4. The high results obtained in the experiments with both Heliaiithus and 

 Catalpa have been given by leaves detached from the plant. Whether leaves 

 still attached to the plant assimilate at equally high rates will depend upon 

 the stomata. The conditions differ in one important respect, since attached 

 leaves have in general to absorb water from the stem against a negative 

 pressure. After detachment the resistance has no longer to be overcome, 

 and, until the vessels become blocked by air or mucilage, detached leaves 

 might well open their stomata more widely than similar leaves still on the 

 plant. 



The magnitude of this effect will vary, however, with different plants, and 

 under different conditions. The most marked effect would be expected in 

 the case of trees ; but with some, especially herbaceous plants, the water 

 supply might be so abundant that even attached leaves could open their 

 stomata to their widest extent. 



Thus Brown and Escombe observed that much more CO2 diffused into 

 detached leaves of Catalpa than into leaves still upon the tree, but the same 

 phenomenon would not necessarily have been observed with Helianthus. 

 The few observations which have been made with the horn hygroscope on 

 leaves of the latter have not revealed differences in respect of stomatal 

 aperture between leaves on and off the plant. 



5. It follows that the lower results obtained by Sachs and by Brown and 



