18 THE STRUGGLE FOR EXISTENCE 



tions existing here are, however, somewhat complicated (see Braun- 

 Blanquet, '28). 



The problem of the influence of environment on competition pre- 

 sents considerable interest, but as yet what we know is very meager. 

 In the majority of cases it is observations of a qualitative character, 

 of which we can give an example here: "The root-systems of the 

 vegetation in the steppes of Southern Russia form, according to 

 Patchossky, three strata. The uppermost one consists of short roots 

 belonging to annual plants which vegetate for a short time. The 

 second, deeper-lying stratum belongs to the essential plants of the 

 steppe vegetable covering, the Gramineae. The third, deepest 

 stratum consists of the vertical stem-like roots of perennial dicoty- 

 ledons (among them the steppe Euphorbia). Usually, the second 

 gramineous stratum dominates. When, however, an immoderate 

 pasturing takes place in a given locality, the gramineous covering 

 begins to suffer and does not produce a vigorous root-system. At- 

 mospheric precipitation can now penetrate to those soil horizons 

 where roots of the dicotyledons are situated, and the latter begin to 

 dominate. As a result appears an unbroken vegetable covering con- 

 sisting of Euphorbia. Analogous results take place in case of increase 

 in yearly atmospheric precipitation. In this case, although the water 

 is energetically absorbed by the second gramineous root stratum, the 

 rainfall is so considerable that a great part of the water penetrates 

 deeper, contributing to the development of dicotyledonous plants. 

 The large dicotyledons act depressingly upon the Gramineae, and 

 they change places in respect to their domination" (Alechin, '26). 



(5) The part which the quantitative relations between species at 

 the beginning of their struggle play in the outcome of competition 

 presents an interesting problem. Botanists do not possess exact 

 quantitative data bearing on this question, and one meets only with 

 considerations of the following kind: When new soils are colonized, 

 if the species concerned do not sharply differ in their capacity for 

 spreading, it mostly depends on chance which species colonizes the 

 given area first. But this chance determines the further colonizing 



Planaria monlenegrina and PI. gonocephala occur in competition with each 

 other, temperature is the factor which governs the relative success and effi- 

 ciency of the two species. PI. monlenegrina is the more successful at tempera- 

 tures below 13-14°C. Above these temperatures PI. gonocephala is the more 

 efficient form." 



