W.F.Loehwing 35 1 



mineral elements in the vegetative organs nourishing them (5, 105). 

 Seeds and fruits are highly selective in the elements which they accumu- 

 late from leaves and stems (5). 



In recent years, agronomists studying the problems of fertilizer place- 

 ment have reported significant differences in response to a given ele- 

 ment with the stage of plant development at which it is supplied (6, 7, 

 12, 16, 19, 32, 33, 39, 41, 45, 48, 50, 72, 97, 101, 105, no, 114, 115, 118, 119, 

 120, 128). Striking differential effects upon specific tissues and the course 

 of ontogeny have been observed. There is evidence, for example, that 

 nitrogen applications at or immediately following anthesis induce re- 

 sponses in reproductive and vegetative organs which are quite different 

 from those observed in plants held continuously at uniform levels of 

 nitrogen supply (12, 33, 47, 101, no, 114, 128). During the early fruiting 

 stage, many species exhibit a high absorptive capacity for and extreme 

 sensitivity to increments or diminutions in nutrient supply (34, 92, 114). 

 Their responses to fertilizers made available at this time are often 

 wholly unlike those supplied at earlier or later stages (132). 



Where marked deviations from ordinary growth patterns are observed 

 following changes in nutrient level at mid-developmental stages, it be- 

 comes of interest to learn which tissues are primarily affected and in 

 what manner, both as to function and structure. Detailed information 

 on these responses is as yet quite meager, but judging from definite 

 responses thus far reported, closer study of them should prove peculiarly 

 productive in extending our understanding and control of fundamental 

 features of plant development. 



The recent work of Rankin (no) on staggered and late supplies of 

 nitrogen in producing differential effects upon the number of spikes, 

 florets per spike, weight and number of kernels per plant in wheat is a 

 case in point (12, 115, 120, 128). Another is the work of Sybil (132) on 

 tobacco in which a shift from low or medium to high nitrogen supply 

 at anthesis produced a leaf structure and organization quite unlike 

 those observed in plants grown at uniform nitrogen levels. It is also 

 worth noting that the form in which nitrogen is supplied is important, 

 as the effects of ammoniacal and nitrate nitrogen may be quite different 

 (154). The yield of fruit produced by a given amount of vegetative 

 tissue can be varied appreciably depending on the form of available 



