224 



inoinent the force then acting can be completely utilized. This 

 assumption as to rainfall is actuall}^ fulfilled over by far the largest 

 part of the European area hithei'to studied by Linsser. 



Of course, we can not speak of absolute quantities of heat or nour- 

 ishing material. We have to do only with their relative distribution 

 during the period of vegetation — that it to say, with the ratio of the 

 quantity of material (/) to the quantity of heat {w). If we con- 

 sider tliait the quantity of material that a definife quantity of heat is 

 able to work up for the use of the plant is directly proportional to this 

 quantity of heat, then the ratio f/iv will ha^e for each plant and phase 

 a certain definite value that may be called the most favorable ratio 

 and for which value the material on hand is completely used up by the 

 heat or active force that is present. If the material that is present 

 is not sufficient for the heat, then f/ir is smaller than this most favor- 

 able value, and in this case the material is completely used up ; but 

 a portion of this heat remains unused and wasted. If, on the other 

 hand, the heat is not sufficient to iise up all the material, then f/tr is 

 loo large and the heat is completely used, but a portion of the material 

 is Avasted. 



The fractional portion of the. annual sum total of heat that is 

 needed to bring a plant up to any stage of vegetation is by Linsser 

 called the " physiological constant "" for that phase and plant, and is 

 constant wherever the plant is acclimatized. The ratio f/w, as com- 

 piled by him month by month for each of his stations, is a local cli- 

 matic constant, which is large Mhen the climate is favorable to the 

 growth of the plant — that is to say, when there is abundance^ of 

 rain — but is small when the climate is more or less unfavorable-to the 

 plant — that is to say, when the summer rains are deficient. 



The vegetation of the Avhole Avorld is, according to Linsser's vieAvs, 

 to be diA^ded into zones (A, B, C, D, E, F), according to the annual 

 distribution of the monthly ratios f/n\ Thus in the highest lati- 

 tudes (Linsser's zone A) and in the greater part of the European 

 region covered by Linsser's researches, there is during the entire year 

 a deficiency of heat, but a sufficiency of moisture and of material to 

 employ all the heat force that is aA'^ailable. In the Steppes of Rus- 

 sia, however, there is a deficiency of moisture during the summer and 

 autumn, and the fraction f/w becomes quite small for the zone B. 

 The other localities that haA^e a wet and a dry period annually may 

 be diAnded into three classes, viz, C, Avhere the drought comes during 

 the months of July and December ; D, where the drought comes dur- 

 ing the months of January and June, or E, Avhere there are two 

 annual droughts, January to March and June to August. This latter 

 arrangement is shown in ^Madeira in the A'egetation of certain kinds 

 of apples. Finally, we may haA'e in zone F a perpetual abundance 



