26 ECOLOGY, PLANT GEOGRAPHY [Bot. Absts., Vol. IV, 



soil development and soil water the latter species having the wider range of tolerance. — The 

 distribution of Thuja occidentalis, while defying explanation on the basis of temperature, water 

 or soil as limiting factors, seems to be explicable on the theory that it has radiated from 

 a limited central area and that migration is still lagging behind ecological conditions. Pinus 

 Banksiana on the contrary is regarded as a pioneer form more tolerant of severe conditions 

 than of competition. This accounts for its absence from areas of better soil development.— 

 Tsuga is regarded as still migrating and hence showing limits by time rather than by factors 

 of its present environment. Notes are also given on the specific ranges of tolerance of Larix 

 americana, Pinus Strobus, Populus balsamifera, P. tremuloides, Acer saccharum, Fagus 

 americana, Ulmus americana, and Betula lutca. [See also Bot. Absts. 2, Entry 17.] — Geo. D. 

 Fuller. 



191. Johnston, Earl S. A simple non-absorbing atmometer mounting. Plant World 21: 

 257-260. Fig. 1. 1918. — A description is given of a mounting that will prevent the absorp- 

 tion of rain and is simpler and less liable to breakage than those previously devised. The 

 principal improvements are the elimination of one of the two mercury valves formerly used 

 and the use of but one tube from the reservoir to the cup. It is claimed that the mounting 

 may be made without employing the services of a skilled glass-blower. Simple but complete 

 directions for its construction are accompanied by a diagrammatic drawing of it as installed. 

 [See also Bot. Absts. 2, Entry 551.] — Geo. D. Fuller. 



192. Kincer, Joseph Burton. Relation between vegetative and frostless periods. 

 Monthly Weather Rev. 47: 106-110. Charts 1-S, jig. 1-5. 1919.— Vegetative temperature 

 defines the potential period of plant growth determined by the spring date when the temper- 

 ature rises sufficiently to permit action by the protoplasmic content of vegetable cells and 

 the date in the fall when it falls below this point and growth ceases. The frostless period is 

 determined by the dates of the last killing frost in spring and the first in autumn. The 

 average frost-free date both in spring and in fall corresponds to a mean daily temperature of 

 from 52° to 56°, and, when away from marine influence, these values are very uniform under 

 both topographic conditions and over large geographic areas. The average frost date can 

 be more accurately determined from the mean temperatures than from the frost records them- 

 selves which are often misleading. — A law of frost occurrence is deduced: "Killing frost occurs 

 each year in spring until the normal mean daily temperature rises to approximately 43°. 

 After this temperature is reached, frost does not occur in all the years, but does occur in more 

 than half the years until the average temperature reaches 54°, which corresponds in data to 

 the average occurrence of the last frost in spring. Thereafter killing frost may be expected 

 with less frequency, or in less than half the years, until the mean daily temperature rises 

 to approximately 63°, after which it is not experienced. This holds good also for fall frost." 

 — Plotting the annual march of temperature and average spring and fall frost dates for suc- 

 cessive belts of 3 C latitude in width from 29° to 49°, between the Rockies and the Appalachians, 

 a series of paralleled curves results in which the line through frost dates is essentially parallel 

 to the vegetative temperature line of 43°, and 9° higher. The lengths of the seasons are widely 

 divergent, the frostless season varying from 20 days from the northern end of the Mississippi 

 Valley to 100 days in the southern. — E. N. Munns. 



193. Korstian, C. F. Life forms, leaf size and statistical methods in phytogeography. 

 [Rev. of: Smith, Wm. G. Raunkiaer's life forms and statistical methods. Jour. Ecol. 1: 16- 

 26. 1913.] Jour. Forestry 17: 328-331. 1919. 



194. MacKay, A. H. The phenology of Nova Scotia, 1917. Proc. and Trans. Nova Sco- 

 tian Inst. Sci. 14: 395-402. 1919. 



195. McGregor, E. A. The relation of irrigation to humidity in a recently reclaimed des- 

 ert. Plant World 22: 45-52. S fig. 1919. — A study of the average monthly humidity of the 

 Imperial Valley during 1916-1918, and of the relation between irrigation and humidity shows 

 that irrigation has little to do with the humidity of the atmosphere. The humidity curve 



