184 The Phenomena of Morphogenesis 



DIFFERENTIATION AS EXPRESSED IN STRUCTURE 



External Differentiation 



One of the most obvious examples of differentiation is that which 

 arises between the ends of a polar axis (p. 116). In all but the simplest 

 axes the structures that are developed at the two ends are quite unlike. 

 The most familiar instance is the differentiation of the shoot and root in 

 higher plants. These two systems are set apart very early, almost at the 

 beginning of embryonic development, and are fundamentally unlike in 

 structure, function, and method of growth. Roots frequently develop 

 from shoots but shoots less commonly from roots. Berger and Witkus 

 (1954) have reported that in Xanthisma texanum the cells of the root 

 always have four pairs of chromosomes but in those of the shoot, some 

 plants have four pairs and some have five. The two types of plants are 

 morphologically indistinguishable. How this difference in chromosome 

 number arises in development is not known, but it is present in young 

 seedlings. 



Another conspicuous instance of differentiation in structure is that be- 

 tween the vegetative and reproductive phases of the life cycle. In its 

 early stages, the plant is becoming established. Its roots and leaves are 

 formed or its vegetative thallus developed, and its career as a food-pro- 

 ducing or food-acquiring organism is begun. Few plants, however, are 

 permanently vegetative. When a certain stage is reached, growth no 

 longer produces exclusively vegetative structures. Flower buds appear 

 at the meristem, or in lower plants reproductive organs of various sorts 

 begin to develop. These usually are formed as the result of internal 

 metabolic changes in the plant, such as the accumulation of carbohy- 

 drates or the production of specific substances. The onset of the reproduc- 

 tive phase, however, is often closely related, as to time and extent, with 

 certain environmental factors, notably light (p. 315). Short-day plants will 

 flower only when the daily period of illumination is relatively short, and 

 long-day ones only when it is longer. Some plants may never flower, and 

 others may do so when they have hardly begun to develop. The balance 

 between vegetation and reproduction may be tipped in various ways but 

 the potency for reproduction is always present in the genetic constitution. 

 This may not always be for sexual reproduction. In species which repro- 

 duce chiefly by vegetative means flowers may be present but fail to 

 function (as in the potato), may be much reduced (as in the banana), 

 or may even be quite absent. Reproduction of some sort obviously is 

 necessary, and the alternation of vegetative and reproductive phases, 

 each essential in the life of the plant, is an important manifestation of 

 differentiation. 



