Growtli ami variation in maize. 115 



of the internodes. Whetlier the approaching: maturity of the sexual 

 organs affects in any way the growth of the plant is a physiological 

 subject worth investigating, hut upon which we have no exact data. 

 It would appear that the rapid elongation of the flower stalk in many 

 plants or of the seed stalks, as in the common dandelion, was in some 

 way dependent upon the reproductive organs. It is possible that the 

 same is true with the corn plant in which case the effect of tasseling 

 would be quite analogous to that of puberty in mammals. 



The facts brought out by these growth data strongly suggest that 

 there are certain well defined cycles in the growth of the corn plant. 

 Owing to the nature of the data, and to the fact that all of the early 

 growth is not included in these measurements, it is not possible to 

 present definite proof of all of these cycles. However, it wiU be worth 

 wliile to indicate their existence, and to show the evidence upon which 

 our conclusions rest. 



In all, we can distinguish four such cycles. The first of these 

 may be called the root growth cycle. This may be described as 

 extending from the beginning of growth until about June 19 in the 

 present data. This period is characterized by the rapid growth of the 

 root system. The evidence for the existence of this cycle is indirect 

 and is based cliiefly upon the behavior of the variation constants during 

 this period. It was pointed out above that there was a marked increase 

 in the relative varial)ility up to .June 19. These changes are probably 

 caused by variation in the time of the establishment of the root system 

 by the various plants. 



The second growtli period may be called the leaf cycle. It is 

 characterized by the development and growth of the leaves. It extends 

 from about June 19 to the l)eginning of tasseling, about July 1. 

 Naturally these growth cycles overlap each other to some extent. The 

 periods given are those particularly characteristic. This cycle is 

 characterized by a steady but niodeiate increase in height. The absolute 

 variability remains practically constant, but there is a very marked 

 decrease in the relative variability with reference to the height of plant. 

 During this period the main growth enei'gies of the plant are directed 

 towards the development of the leaf system, and growth in height is a 

 relatively secondary matter. 



The third growth period is the tassel cycle, and is characterized 

 by the rapid elongation of the upper internodes of the plant. This cycle 

 extends from the beginning of tasseling, about July 1, until the blooming 



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