266 BOTANICAL GAZETTE [april 



time lost when the plant is watered and rebalanced or to the falling 

 of sunshine directly upon the plant. The drop in the curves between 

 February and March i8 just after noon is due to the shading of the 

 plant from the direct afternoon sun by an intervening house. 



The effects of cool, cloudy, moist days are well brought out in the 

 curve of Phaseolus vulgaris, where such conditions prevailed the first 

 half of the week, tending to check transpiration; w^hile the brighter, 

 warmer, drier, latter half of the week increased the loss of water. 

 The same differences are seen equally well in the curves of Pelargo- 

 nium domesticum and P. peltatum, Fuchsia, and Tropaeolum; and are 

 more strikingly shown in those of Senecio mikanioides, Lupinus amis, 

 Zea Mays, Senecio Petasitis, Cucurhita Pepo, Lycopersicum escukntim, 

 and Abufilon striatum; but the last-named plant and Chrysanthemum 

 were at no time in brilliant sunhght. 



In general the transpiration curve supplements the tabulated 

 results, showing in addition the effects of physical conditions. The 

 important fact brought out by the curves as a whole is the extreme 

 sensitiveness of transpiration to even sHght changes in external condi- 

 tions, a fact already mentioned above in connection with the varia- 

 tions shown under "standard" conditions. So great, indeed, is this 

 sensitiveness that it seems out of all proportion to the direct physical 

 changes and suggests the possibiHty that the relation between condi- 

 tions and transpiration is not purely physical, as it is apparently now 

 considered by some students, but is indeed one which involves the 

 action of the conditions as stimuh. But this is a separate matter and 

 I have been concerned merely with transpiration as a fact, quite 

 regardless of its explanation. Furthermore, I have not tried to sepa- 

 rate the influence of the conditions nor to show, when several cooperate^ 

 which produces the greatest effect. I have taken them as they came; 

 their separation is a separate investigation. 



We consider now the results of this study from the practical point of 

 view of the teacher, as to which plants are best for the demonstration 

 of transpiration. While the columns designated "M^HG average' 

 give some plants a greater average transpiration, the columns o| 

 "M^HG minimum and maximum" really offer a better choice of 

 material, because they show the great range possible for each plant. 

 Helianlkus annuus stands first in amount of transpiration, but it is open 



) 



