26o BOTANICAL GAZETTE [apeil 



numerous instruments requisite for obtaining averages from several 

 plants. Aside from this, however, the results are more representative 

 than may appear at first sight. On the one hand the plants were care- 

 fully chosen from a considerable number as typical specimens of their 

 kinds, and on the other, according to Quetelet's law-, any single 

 plant taken at random is more likely to fall close to the mean than far 

 from it. Hence, taking our thirty plants collectively, while some of 

 them may de\iate considerably from the mean of their kind, the great 

 majority must lie more or less close to it. 



Although the table of figures expressing the total quantities of 

 transpiration has its value, it cannot be made to throw light upon the 

 effect of external conditions upon transpiration. In consequence 

 graphs made of each plant under greenhouse conditions were plotted 

 from the records of the autographic instruments. In all cases they are 

 for six days, the day and night being indicated on the base line. Each 

 abscissa space represents 2 J hours of time; each ordinate space m 

 the transpiration curve equals either i, ^, or 2 grams of water lost, 

 as indicated in each case, the difference being necessary to bring the 

 curves of the plants wdthin practical ranges; one vertical space in the 

 temperature curve represents 5°; in the curv^e of moisture, i per cent, of 

 saturation; and in that of light, 5, 2.5, or 10 per cent, of intensity. The 

 curv'e of moisture has been inverted into one of dryness so that all 

 physical conditions tending to increase transpiration have upward 

 turns, and vice versa. The curve of light shows the intensities avail- 

 able to the plant, not what the plant actually used; this varies with 

 the structure and the internal physiological processes. 



As indicated above, these graphs have two values. The first is to 

 show under what external conditions the figures for ordinary condi- 

 tions given in the table were obtained. It is obvious that these values 

 are profoundly affected by the conditions prevailing when the experi- 

 ment was in progress. The second value is to present as clearly and 

 graphically as possible the relations of the rate of transpiration to 

 external conditions. It is not supposed that these graphs can bring 

 out any knowledge of transpiration new in the abstract, but they do 

 show this relation with gratifying clearness for our most familiar 

 plants, and in a form which, it is hoped, will make them valuable in 

 educational work. 



