Botany 125 



as well as of generally prevalent conceptions, regarding the relations that exist 

 between natural plant distribution and climatic conditions in the United States. 

 The first section deals with the facts of geographical distribution with reference 

 (1) to the main general vegetational types of the area considered, (2) to selected, 

 ecologically conformic groups of plants, and (3) to selected individual species. 

 These data are presented by charts showing the geographical areas actually oc- 

 cupied by the various types, etc., considered. 



The second section deals with the climatic conditions that are known to influ- 

 ence plant activity. These conditions first receive attention from the physiological 

 point of view, and a discussion is given regarding the physiological principles 

 upon which the quantitative etiological aspect of biatic distribution and of ecology 

 in general may eventually be worked out. Then follows a presentation of the 

 climatic areas of the United States, based upon those features for which quanti- 

 tative data are available, and upon various methods for computing these data 

 so as to give promising climatic indexes. An important feature of many of these 

 index values and a new one for such studies as this, is the computation of the 

 indexes for the period of the average frostless season. All these different kinds 

 of climatic index values are presented in tabular form and also cartographically. 

 By the latter method of presentation the area of the United States appears as sub- 

 divided into climatic zones or provinces, each kind of index having its own 

 set of provinces. In certain cases (notably for the very important Transeau 

 moisture-ratio of precipitation to evaporation) two or more different kinds of 

 climatic indexes are combined <to give a compound index. 



The remaining section of the book gives the results obtained by superimposing 

 each of the vegetation charts on each climatic chart and determining the maximum 

 and minimum values of the climatic index for the geographical area occupied 

 by the vegetational feature, in each case. These climatic limits for each particular 

 vegetational area are shown by tables and also in the form of diagrams that 

 graphically set forth the climatic limits for each area with reference to the total 

 range, within the United States, of each index value. The authors emphasize 

 the fact that they are dealing only with actual correlations between certain cli- 

 matic indexes on the one hand, and the extent of certain vegetational areas on 

 the other, and they point out that the correlations discovered by their method 

 do not necessarily indicate corresponding actual controls of vegetational distribu- 

 tion. The book deals primarily with actual facts and the quantitative correla- 

 tions that exist, and the discussions aim to open up this important field of science 

 in a logical and quantitative way, rather than to advocate any particular theories 

 about the climatic control of plant distribution. From the results secured it is 

 especially clear that the study of climatic control of biotic ranges can not progress 

 far as long as the different kinds of climatic features are considered separately; 

 each climatic feature must be considered as influencing organisms in a specific 

 way only in the presence and with the concurrent influence of all the other effec- 

 tive climatic features. The climatic complex acts as a whole, and its various 

 component features are generally interdependent with regard to their powers to 

 influence the life processes. It is pointed out that this feature of environmental 

 influence renders the study of etiological distribution just as difficult as it is 

 important. 



No. 194. SHREVE, EDITH B. The Daily March of Transpiration in a Desert Peren- 

 nial. Octavo, 64 pages, 1 plate, 27 figures. Published 1914. Price $0.75. 



A study of Parkinsonia microphylla was undertaken with a desire to determine 

 some of the means by which a non-succulent desert perennial passes the drought 

 periods which occur twice each year in the climate in which it flourishes. The 

 results found can be divided into two classes : the external means and the internal. 

 The external means involves the lessening of the transpiring surface caused by the 

 dying and dropping of leaves, twigs, and sometimes whole branches without injury 

 to the life of the plant. The internal means involves the lessening of the water 

 loss per unit area during the forenoon, when the evaporating power of the air is 

 increasing. This decrease is accompanied by a lowering of water content of leaves 

 and twigs by a partial closure of stomatal openings, and by a rise in leaf tempera- 

 ture. The transpiration rate was greatly decreased when the soil moisture be- 

 came low. 



