306 Morpho genetic Factors 



A second group are the chemical factors, which derive their impor- 

 tance primarily from their participation in the chemical processes going 

 on in the plant. Some substances, notably those in mineral nutrition, 

 come into the plant from the outside, but many originate internally as 

 products of the plant's metabolism. Especially important in morpho- 

 genesis are the various growth substances. 



A third group of factors, the genetic ones, may also be regarded as 

 part of the internal environment. Here are to be considered the genes, 

 permanent and self-perpetuating; the chromosomes, which may have 

 certain morphogenetic effects apart from the genes they contain; and 

 the cytoplasm, the intermediary between genes and developmental 

 processes. These factors, though doubtless effective because of their 

 physical or chemical character, are difficult at present to reduce to such 

 terms and are best considered by themselves. 



The effects of these various factors on development are complicated 

 by the fact that they are operating on an organized living system which 

 tends to regulate its activities in conformity to a specific norm. Three 

 consequences of this should be borne in mind: 



First, a given factor does not lead directly to a given result but serves 

 instead as a stimulus or evocator that sets off a reaction in the organism. 

 What this will be depends on the state of the system at the time. The 

 effect of light on a photographic plate is easily predictable, but its effect 

 on a plant depends on the part of the plant concerned, the age of the 

 plant, and its physiological condition. 



Second, the effect of one factor may be greatly modified by another. 

 The photoperiodic effect of light, for example, may depend in a given 

 case on the temperature of the environment, so that one factor may 

 sometimes be substituted for another. Although the essence of good 

 experimental work is to deal with only one variable at a time, this often 

 is impossible in morphogenetic experiments (and in biology generally) 

 for no one factor can be studied entirely independently of the others. 

 What it will do depends on the rest of the environment and on the state 

 of the organism. 



Third, the organized system is not a constant one but tends to change 

 in character from one phase in its life cycle to the next and from one 

 region of the body to another. The potency of a cell (the repertoire of 

 developmental possibilities open to it), high at first, is reduced as the 

 cell grows older. Doors continually close behind it, so to speak. The 

 reactivity of a cell (the response it will make to a given environmental 

 change) also is different at successive developmental stages. Both 

 potency and reactivity may be unlike in different parts of the organism. 



An investigation of the effects of various factors on plant develop- 

 ment, particularly environmental ones, though not as simple as might 



