MARYLAND WEATHER SERVICE 33 



etc., — are merely protective against sudden changes of temperature, 

 which are more injurious than the low temperature in itself. There 

 are, nevertheless, many plants whose protoplasm is not capable of 

 withstanding a temperature of 32°, or even a few degrees above this, 

 except in the resting state of the s>^'<\, in which the water content of 

 the protoplasm is low. 



Considerations regarding the length of the growing season and the 

 effect of the annual march of temperature upon the growth and peri- 

 odic phenomena of plants are of great theoretical and practical in- 

 terest. The attempt has been made repeatedly to establish a definite 

 mathematical relation between the stages in the annual march of 

 temperature and the seasonal changes in the growth and periodic 

 phenomena of plants. These attempts constitute the science of 

 phenology, for the history of which the reader is referred to a recent 

 publication by Professor Abbe*. The principal attempt of phenology 

 has been to show that the same plant species reaches the same stage 

 of development or exhibits the same periodic phenomena in different 

 locations, or in different years at the same location, when it has re- 

 ceived a particular total amount of heat, or a particular proportion 

 of the total heat of the growing season. This attempt has been beset 

 with many difficulties, as are all attempts to establish biological laws 

 on a priori grounds. On the side of the temperatures it is necessary 

 to establish an arbitrary starting point from which to begin the ad- 

 dition of the number of degrees ; it is necessary to use the degrees of 

 temperature as observed in the shade or else to secure insolation 

 temperatures, which cannot lie reliably measured; it is necessary to 

 ignore the fact that some of the light rays falling upon the foliage are 

 converted into heat which is available to the plant. On the side of 

 the plant it is necessary to ignore the physiological difference between 

 the germination of annuals and the leafing-out of perennials; it is 

 necessarv to ignore the fact that a given ten degrees of the thermo- 

 nietrical scale are not physiologically equivalent to any other ten 

 degrees above or below these ; and above all it is necessary to ignore 

 the I act of the influence upon the plant of many other external factors 



*Abbe. Cleveland. A First Report on the Relations between Climates and 

 Crops. Bulletin 36, United States Weather Bureau, Washington, 1905. 



