IN THE DIVIDING ROOT-TIP CELLS OF THE ONION. 35 



Not only would we look for rhythm, as caused by the complex of 

 envh-omnental factors, but the internal organization of the plant permits 

 response at one time or season, but not at another. That is, besides 

 the daily response in mitotic and growth rhythms, due chiefly to ex- 

 ternal influences, there is a seasonal rhythm due chiefly to internal 

 organization. Thus in February and March the cured onions, which 

 have been stored through the winter, sprout very readily upon being 

 given moisture and light; but in August the same type of onion, as was 

 earlier reported, is hard to awaken to growth. (See p. 27.) Then, too, 

 each individual tissue of each individual animal or plant would be 

 expected, under a definite complex of environmental factors, to present 

 its own specific train of mitotic phenomena, the parallelism in re- 

 sponse being governed in such cases by the degree of constancy in the 

 en\aronment-complex and in the genotypic constitution of the tissues 

 compared. 



B. HEAT FACTOR IN GROWTH. 

 (a). GENERAL. 



Heat is known to exert an important influence upon the velocity (see 

 p. 38) of chemical reactions, and also upon the reaction-rate or strength 

 of practically all of the measurable physical forces known in both the 

 inorganic and the organic worlds. Growth (bulk-increase and mitosis), 

 which is a complex of chemical and physical reactions, can take 

 place only under appropriate temperature-conditions. Other things 

 being equal, the growth response of a specific plant is specific for a given 

 temperature. Many experiments have been conducted upon the rate 

 of growth for the purpose of working out physiological constants for 

 given and various situation-complexes of nature and nurture. So far 

 as temperature-relations are concerned, there have been found cardinal 

 points, specific temperatures, at which growth in a specific plant 

 responds at its mimmum, its optimum, and its maximum rates. As a 

 rule, these points are found to vary from slightly above zero to approxi- 

 mately 50° C. 



ib). PHENOLOGY. 



The phenologists have found a certain relationship between the quan- 

 tity of heat (that is, the number of centigrade-degree days) and the stage 

 reached by a given plant in its development from the dormancy of mid- 

 winter. Linsser,^ in 1867, attempted to formulate this relationship. 

 His conclusions were based upon the theory that a definite quantity 

 of heat is required in order to affect the internal reactions necessary to 

 reach a definite developmental stage; regardless of whether this quan- 

 tity be distributed over a long or a short season, its end effect was 

 thought to be the same. In general, phenology is an attempt to har- 

 monize the known facts of energy transmutation and conservation in 



1 Linsser, Carl, "Die Periodischen Erscheinungea des Pflanzenlebeas in irhem Verholtniss zu 

 den Warmeerscheinunzen." Mem. Acad. Sci., St. Petersb., Ser. VII, Vol. XI, No. 7, 1867. 



