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INTRODUCTION TO PLANT ECOLOGY Te 
a subordinate place, and therefore named “ secondary ”; these 
may be gradually replacing the dominant forms. Lastly, 
there may be in any given area plants which depend for their 
existence on the presence of the dominant species. A good 
instance of this is the undergrowth of a beech wood, which 
for the most part is dependent on the decaying vegetable 
matter of the beech leaves. The plants constituting the 
undergrowth of a beech wood in many cases would not be 
found in a pine wood, where there would not be enough 
humus for them. : 
Kcology investigates this relation of plants towards each 
other ; it tries to explain how it is that certain plants are 
successful in the struggle for existence, it may be owing to 
more rapid growth, quicker germination, or longer duration. 
It shows which plants are social in habit, growing together in 
groups, as firs, wood anemones, grasses ; which are sparsely 
intermixed, and which are solitary. 
In Chapters IV. and V, these Plant Associations are dis- 
cussed and suggestions made for field work. 
BIBLIOGRAPHY. 
‘““Okologische Pflanzengeographie”: Warming. Translated by Knob- 
lauch. There is as yet no English translation. 
Natural Science, February, 1899, vol. xiv., No. 84. ‘‘On the Study of. 
Plant Associations” : Robert Smith. 
