PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



87 



dormant period; if it occurred later than this, 

 it could be inferred that the inflorescence has a 

 very rapid rate of growth and differentiation. 



Flowering 



There have been many investigations into the 

 control - of flowering in grasses and, in general, the 

 morphogenetic events associated with flowering 

 are similar throughout the family. The apical 

 meristem of the tiller undergoes a transformation 

 and becomes completely involved with the pro- 

 duction of the inflorescence which is thus termi- 

 nal. After flowering, the tiller normally dies, 

 although the plant as a whole might continue 

 through the production of a new crop of tillers 

 at the base of each flowering culm. There are, 

 indeed, two ways in which a grass plant can 

 achieve perenniality. On the one hand, not all 

 the tillers might become reproductive at the 

 same time — this would appear to be the situation 

 in Astrebla where underground rhizomes with 

 many potential meristematic sites provide a store 

 of future tillers. On the other hand, axillary 

 tillers from the base of flowering culms may 

 grow out after flowering as a new crop of vege- 

 tative tillers. 



It is quite clear from the review of Evans (5) 

 that there are many forms of environmental 

 response in the Gramineae. Some require long 

 days, others short days, still others require other 

 specific photoperiodic conditions for flowering, 

 and a further group are insensitive to photoper- 

 iod. Similarly, some grasses have an inductive 

 need for cold or short days before they are capa- 

 ble of responding to photoperiod. Still others 

 may need to undergo a certain amount of vege- 

 tative growth before they will respond to any 

 conditions which favor flowering (2). From all 

 that is known, it is clear that while the morpho- 

 genetic events of flowering may be rather similar 

 in all grasses, the environmental conditions which 

 stimulate the flowering are many and appear to 

 operate at different points of a pathway or even 

 on different pathways leading to the final floral 

 stimulus. 



Jozwik (7), in his work on Astrebla, sheds 

 little light on the environmental control of flow- 

 ering in the genus except to show that, in gen- 

 eral, the inflorescences emerged more rapidly 



at shorter photoperiods (10 hours compared with 

 12 or 14 hours). He concludes that photoperiod 

 plays a significant part in the development of in- 

 florescences but appears to have little effect on 

 the initiation of floral apices. Both field and 

 laboratory experiments in which the apices were 

 dissected would help to answer the questions 

 posed by these results. 



In discussing the results, Jozwik (7) claims 

 that close regulation of flowering by photoperiod 

 would be a distinct disadvantage in the mitchell 

 grasses. Moisture is the main climatic factor lim- 

 iting their growth. Although growing periods 

 commonly occur within a definite season, the date 

 of onset of rain varies widely from year to' year. 

 In addition, adequate rainfalls can occur at any 

 time of the year, and field observations suggest 

 that flowering can take place at these times. Thus, 

 it seems that selection has favored nonspecific 

 flowering requirements just as it has favored non- 

 specific dormancy requirements, allowing the 

 plant to take full advantage of favorable condi- 

 tions when they occur. Astrebla appears to have 

 an exploitive physiology and this could be the 

 case for many of the perennial grasses of the 

 Australian arid and semiarid areas. 



Such a conclusion may not apply to all grasses. 

 Some preliminary work we have undertaken on 

 the control of flowering in Danthonia caespitosa 

 Gandich. from the southern, predominantly win- 

 ter-rainfall areas, suggests that a period of water 

 stress acts as an inductive stimulus to flowering 

 in long photoperiods. It is possible that such a 

 response is linked with abscisic acid production. 

 Like many grasses, Danthonia flowering is re- 

 duced by exposure to very high temperatures. 

 This is a feature of likely adaptive significance 

 in that it reduces seed formation in the hottest 

 months of the year and protects the plant from 

 the physiological strain of inflorescence and seed 

 production at a time of year when water may be 

 limited. 



Conclusion 



The major morphogenetic events which have 

 been discussed briefly in this review are all rele- 

 vant to understanding and promoting plant wel- 

 fare which, I propose, is basic to the productivity 

 and well-being of the range. It should be clear 



