It PLANT SUCCESSION AND CROP PRODUCTION 



ditional facts unknown or overlooked postpone the final rounding 

 up of the field into a completed work. This is as it should be with 

 any good subject. If we try to classify the experimental work in 

 this field we may at present say no more than that the experiments 

 always take one form, namely, the observation and analysis of the 

 factors causing an incre^ase in activity, and those which diminish 

 it and finally stop it entirely as limiting factors become operative. 



The external and internal changes in structure may be sum- 

 marized as ephemeral and continuous. The former continue only 

 as long as the control conditions last. These affect only the most 

 plastic tissues and organs of the plant. The continuous variations 

 produce permanent effects upon the plants or upon the functioning 

 of the tissues and organs. In a rapidly varying stock such perma- 

 nent variations, no matter how induced or perpetuated, are called 

 mutants. This brings one to the verge of physiological ecology, the 

 adjoining field in this direction being genetics. 



B. Synecology 



Synecology deals with groups of plants and studies their de- 

 velopmental activity in mass. It is more geographic than physio- 

 logical. The mass responses are rarely, if ever, the simple sum 

 of the changes of activity of individuals, but more likely a ratio or 

 a quotient. The results of field studies of plants in mass are for 

 this reason different from the results of studies of individual plants. 

 In synecology the plant association as a whole is treated with rela- 

 tion to the factors of the environment which influence its develop- 

 ment. Studies in associational ecology are largely in the observa- 

 tion stage still. Accurate records, according to the strictest methods 

 of research, are made in several ways. The progress of the develop- 

 ment of the vegetation, or the plant successions, may be recorded 

 by means of photographs, soil and water analyses, besides various 

 instruments for keeping constant records of temperature and light 

 changes. These records, when properly put together, tell the story 

 of plant development in detail. The proof of plant successions, 

 while not at all necessary to the hypothesis, may be seen by re- 

 visiting reserved areas where intensive studies have been made. 

 After the lapse of sufficient time, the changes in the appearance of 

 the landscape is marked, but without the use of photographic rec- 

 ords is not nearly so convincing. 



