62 THE PHYSIOLOGY OF STOMATA. 



This, however, is probably not true, since positive change in transpiration 

 rate in the same amplitude may occur without stomatal change, or the rate 

 may continue to increase after cessation of stomatal opening. The limita- 

 tion of the transpiration rate at its maximum in the daily program might also 

 be interpreted as due to the maximal stomatal opening but for the same 

 objection, the truth of which is indicated by Brown &Escombe's observations 

 on Helianthus and my own on Fouquieria. 



The rate of transpiration in the plants here studied also varies in successive 

 time units under constant external conditions, and without illumination, as 

 was recently observed in certain other plants by Curtis (1902). The rates are, 

 however, not correlated with stomatal movements, which are very slow, if 

 they occur at all, in darkness. The slight normal opening might be called 

 upon to interpret a commonly occurring rise in transpiration rate after mid- 

 night or somewhat later, but it is impossible to believe that the lowering of 

 the transpiration rate, after the hour of the maximum of induced rhythm 

 (see, e. g., figs. 21, 22, and 23), is caused by stomatal regulation. 



The possibility of such fluctuations occurring independently of stomatal 

 changes makes it impossible that in these plants stomatal regulation of an 

 effective kind maybe said to take place. Livingston sug- 

 gested that the regulation which he believed to occur in a 

 desert species of Euphorbia, resulting in the maximum 

 transpiration rate being reached before the maximum 

 evaporation rate (of water from a porous shell) may per- 

 haps be due, among other things, to stomatal activity. I 

 have examined the stomata of this plant with a view 

 to obtaining evidence on this point, but to my surprise I 

 find very little observable stomatal movement. The cuticle of this plant is 

 very thick indeed, and the upper and under walls of the stomata are similarly 

 indurated. So thick are the walls of the stomata that but a small amount of 

 movement seems possible. Fig. 20 shows the stomata of Euphorbia sp. in 

 transverse section. The profile of one guard-cell is shown. The distribution 

 of the stomata (upper epidermis, 75 to 100 per square millimeter; lower epi- 

 dermis, 1 50 to 1 75 per square millimeter) would indicate that the leaf move- 

 ments might be more effective if they occur at the proper times, thus lending 

 weight to Livingston's further suggestion that these movements may be 

 regulatory, and in this way be a factor in determining the rates of relative 

 transpiration. 



Stomatal movement, correlated with that of the leaf or leaflet, may, how- 

 ever, occur. Wilson & Greeman (1892) report that in Melilotus on a "very 

 hot morning" the stomata of the vertically placed leaflets were "all without 

 exception" closed, while under alum and ice screens, with a temperature of 8 

 C. lower than without, the stomata were "well open." The authors believe 



