NUMBER- WORK IN NA TU RE-STUD V 7 I 



to resist this strain by reducing the leaf surface exposed to the 

 sunshine. 



3. Temperature. — The average temperature for the five months 

 for thirty-one years is 66 degrees. For the year 1901 the average 

 temperature for the same month is 68 degrees. There was actually, 

 however, an excess of 6 degrees during this time, as shown by the 

 Weather Bureau records. This condition meant, also, not only a 

 possible increase of plant activity ; it meant more than the normal 

 evaporation from the soil which would tend to cut off the water 

 supply from the plant. 



4. Variation in intensity of sunshine. — On the twenty-first day 

 of each month the distribution of a given beam of sunshine at noon 

 is proportional to the following areas : May, 108 ; June, 105 ; July, 

 107; August, 115. These results are obtained approximately by 

 the use of the skiameter. The intensity of the sunshine varies 

 inversely with the areas of distribution. In the month of June, 

 when the intensity of sunshine is greatest, the average cloudiness 

 is 32 per cent.; in 1901 the average cloudiness was 31 per cent. 

 Therefore the withdrawal of the friendly cloud shelter by the amount 

 of I per cent, in the month when the intensity was greatest served 

 still further to increase the stress laid upon the plant in the year 

 1901. 



These causes all happened to combine directly in this particular 

 year to menace the future of the verbena. They also operated indi- 

 rectly, so far as they favor other plants that know how to get along 

 with the reduced amount of moisture and the increased amount of 

 sunshine and heat. 



The botany of the verbena, therefore, for this particular year, 

 becomes chiefly a study of the various devices of leaf, stem, and root 

 by which this plant is able to maintain itself against all these unfavor- 

 able influences, which the work in number shows to be actual and 

 definite forces of enormous power. 



As an illustration of the point, already urged, that the form side 

 of the subject should be studied as the image-growth proceeds, it 

 will be found by an examination of the lessons that the following 

 processes have been involved, which should be formulated or tabu- 

 lated in any convenient fashion (and learned) as the study proceeds : 



1. Reading and writing of numbers up to and beyond six places. 



2. All the fundamental operations. 



