364 Morphogenetic Factors 



often a radical redistribution of substances in the plant body. The plant's 

 life history is composed of such progressive steps. This concept is of 

 importance for morphogenesis and perhaps especially for its chemical 

 aspects. 



The subject of chemical factors in plant life, and particularly of the 

 biochemistry of metabolism, is one of the chief concerns of physiology, 

 but the only aspect of this field germane to the science of morphogenesis 

 is the somewhat limited portion of it which deals directly with the rela- 

 tion of chemical substances to development. The present discussion will 

 necessarily treat this subject in nothing more than a very brief and 

 general fashion. One chapter will be devoted to the role of the elements 

 and the compounds that are primarily significant for their role in nutrition 

 rather than in morphogenesis. A second chapter will deal with those 

 substances that, even in very minute amounts, have been found to exert 

 profound effects on growth and development and are commonly called 

 growth substances. 



ELEMENTS 



It has long been known that only a few of the chemical elements are 

 essential for plant life. These, in addition to carbon, oxygen, hydrogen, 

 and nitrogen, are sulfur, phosphorus, calcium, magnesium, potassium, and 

 iron, together with a number of others, notably boron, zinc, copper, 

 manganese, cobalt, and a few others which, in very small amounts, are 

 essential for the nutrition of most plants and are known as trace elements. 



Several of the elements, or simple compounds of them, have been 

 found to have some effect on development and are thus of morphogenetic 

 importance, though except for nitrogen this is relatively minor. 



Nitrogen. This element is of outstanding significance in many ways. 

 It is an essential constituent of all proteins and thus of protoplasm, and 

 from its presence in the nucleoproteins it is concerned in the production 

 of new living stuff and thus in all growth and reproduction. 



That nitrogen tends to increase the vegetative growth of plants has 

 long been known, but it may also have certain specific effects on their 

 structure. Burkholder and McVeigh (1940) grew maize (both inbred 

 and hybrid ) with varying applications of nitrogen in sand culture. Where 

 nitrogen was abundant, as compared with plants where this was limited, 

 meristems were better developed, length and diameter of stem were 

 greater, cell size and cell number increased as did the size and number 

 of the bundles, there was greater differentiation especially in the phloem, 

 and both sieve tubes and vessels increased in diameter (Fig. 17-1). 



Plants growing with little available nitrogen tend to be woodv and to 

 have thick cell walls, presumably because much of the carbohydrate is 





