224 
moment the force then acting can be completely utilized. This 
assumption as to rainfall is actually fulfilled over by far the largest 
part of the European area hitherto 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 (7) to the quantity of heat (w). If we con- 
sider that the quantity of material that a definite 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 7/w will have 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/w 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 use up all the material, then f/2 is 
too large and the heat is completely used, but a portion of the material 
is wasted. 
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 {/w, as com- 
piled by him month by month for each of his stations, 1s a local cli- 
matic constant, which is large when 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 whole world is, according to Linsser’s views, 
to be divided into zones (A, B, C, D, E, F), according to the annual 
distribution of the monthly ratios f/w. 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 1s during the entire year 
a deficiency of heat, but a sufficiency of moisture and of material to 
employ all the heat force that is available. In the Steppes of Rus- 
sia, however, there is a deficiency of moisture during the summer and 
autumn, and the fraction f/7# becomes quite small for the zone B. 
The other localities that have a wet and a dry. period annually may 
be divided into three classes, viz, C, where 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, where there are two 
annual droughts, January to March and June to August. This latter 
arrangement is shown in Madeira in the vegetation of certain kinds 
of apples. Finally, we may have in zone F a perpetual abundance 
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