48 THE RELATION OF DESERT PLANTS TO 



phenomenon may be brought about to some extent through the action 

 of the green chlorophyl in absorbing heat and thus increasing evapora- 

 tion from the leaves. This is probably not an important factor, how- 

 ever, since such rises in temperature can not be very marked. It is 

 probably brought about mainly by some physiological change in the 

 plant, effective during certain hours, which reduces transpiration to a 

 greater degree than would be brought about by the night conditions of 

 lower temperature and absence of light, as these affect mere physical 

 evaporation. 



This physiological activity of the plant is perhaps mainly the 

 response of the stomatal mechanism.* In this plant it may also be due 

 in part to the nyctitropic movements of the leaves, which, during the 

 hours of darkness or of weak light, fold up closely against the stem 

 and overlap one another so as to decidedly reduce the exposed surface. 

 Lastly, it is possible that the physiological retardation of transpira- 

 tion may be due to some periodic change in the permeability to water 

 of the protoplasm of the plant tissues. This might occur in the roots, 

 which, from the experiments of many authors on the subject of root 

 pressure, seem to show a periodicity in absorptive rate, or it might per- 

 haps occur in the mesophyl of the leaves themselves. No evidence is 

 at hand regarding either of these suppositions. 



In order to facilitate the study of these periods of high and low 

 rates of relative transpiration, the average ratio for the whole period 

 of the experiment has been found and has been plotted on the ratio 

 curve as a horizontal line with a constant ordinate equal to the average 

 ratio, which is 0.027. The average ratio was obtained by merely 

 summing the partial surfaces which are included in the quadrilat- 

 erals bounded by each pair of adjacent ordinates, the curve and the axis 

 of abscissas, and then dividing this total area or integral of the curve 

 by the last abscissa, which represents the entire time period of the 

 experiment. The points of intersection of this line of the average ratio 

 with the ratio curve itself are to be considered as the limits of the 

 physiological periods just noted. Since no withering of the leaves 

 occurred while these observations were being taken, it follows that the 

 plant did not suffer from lack of water during the period of the experi- 



*Burgerstein (1904, p. 32) agrees with previous writers that the condition of the 

 stomata, whether open or closed, etc. , may usually be judged by measurements of the 

 rate of water loss, ' 'denn ist bei einem Blatte die epidermoidale Transpiration gegring, 

 so wird die Grosse der Gesamtverdunstung, die in diesem Falle hauptsachlich auf 

 Rechnung der stomataren Transpiration kommt, bis zu einem gewissen Grade propor- 

 tional sein dem Offnungszustand der Spaltoffnungen, so dass man bei relativ hohem 

 (durch Wagung ermittelten) Transpirationswert, auf Offnung, bei sehr geringer tran- 

 spiratorischer Leistung auf eine mehr oder wenger vollkommene Clausur der Stomata 

 schliessen kann. " 



