PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



83 



almost 30 percent of Australia's arid lands. Their 

 grass ground cover is well developed, in the north 

 being dominated by Astrebla and in the south by 

 Danthonia, Enneapogon, Stipa, and Eragrostis. 

 This short survey of the genera of Australian 

 range grasses emphasizes the major division into 

 summer rainfall northern types and winter rain- 

 fall southern types. Only the mitchell grasses 

 form a grassland community of considerable eco- 

 nomic importance. With the exception of the 

 hummock grasses, the remaining genera are as- 

 sociated with woodland or shrubland communities 

 and in these they provide important contribu- 

 tions to the fodder yield. 



Germination And Establishment 



Germination of the grass caryopsis involves 

 first the imbibition of water, and this is followed 

 by growth of the coleorhiza, a sheath of cells sur- 

 rounding the radicle. The coleorhiza frequently 

 develops cell outgrowths resembling root hairs, 

 and, after a day or so, the radicle pushes through 

 this sheath and down into the soil. Soon after 

 emergence of the radicle, the coleoptile appears 

 and through this grows the first leaf. With the 

 appearance of this leaf the plant is potentially 

 self supporting, and further growth ensures com- 

 plete establishment. 



The earliest stage of germination, the uptake 

 of water, is reversible but once the morphogenetic 

 events of root and shoot growth have started the 

 plant is committed to growth and, at this stage, 

 is particularly vulnerable to environmental ex- 

 tremes, especially drought. 2 Because of this vul- 

 nerability of the seedling, natural selection favors 

 those species which germinate only after signifi- 

 cant rains, that is, those species which have 

 evolved germination control mechanisms. 



Literature on germination and its control has 

 grown massively over a number of decades. Cer- 

 tain conditions of moisture, temperature, aeration, 

 and light are essential for the germination proc- 

 ess, but even when these are provided and con- 

 ditions are optimal, growth may not ensue be- 



* During discussion, attention was drawn to the fact 

 that some North American range grasses, for example, 

 the introduced Agropyron desertornm (Fisch.) Schult. 

 are capable of surviving dessication even after emergence 

 of the radicle. 



cause the seed is dormant (12). Several mecha- 

 nisms by which dormancy can be achieved are 

 given below. 



1. Impermeable seed coats. These may be im- 

 permeable to water, oxygen, or both; germina- 

 tion is prevented until they are altered either bio- 

 logically or mechanically. 



2. Immature embryo. In these the embryo must 

 undergo further growth and development after 

 imbibition before germination appears. 



3. Dry storage after-ripening. Fresh seed may 

 fail to germinate until after a period of storage 

 at ambient temperatures. This after-ripening is 

 common in grasses and could be associated with 

 gradual changes in the relative proportions of 

 growth promoters and inhibitors. 



4. Specific environmental requirement. Some 

 seeds need preehilling or exposure to light before 

 germination can take place. These conditions 

 are necessary as a triggering mechanism to pro- 

 mote germination. 



Seed dormancy exhibited by many Australian 

 range grasses has been a deterrent to further 

 work on their growth and development. Unpub- 

 lished investigations 3 on two species of Stipa 

 demonstrate the presence of a growth inhibitor 

 in the fresh seeds with properties similar to syn- 

 thetic abscisic acid. On storage at room tempera- 

 tures (between 20° and 25° O), the levels of this 

 inhibitor decline, and there is an associated in- 

 crease of a growth promoter. In these two species, 

 there appears to be a control mechanism which 

 is based on the balance of growth regulators, 

 both promoters and inhibitors. A water leachate 

 from fresh seeds of these plants contains a strong 

 growth inhibitor. Under natural conditions it is 

 possible that germination control is exercised 

 through a leaching process, inhibitor levels only 

 being reduced to a low enough level after signifi- 

 cant rains — rains which can support the con- 

 tinued growth necessary for full establishment. 

 Themeda austral/s (R. Br.) Stapf. also contains 

 an inhibitor in the fresh seed, but this has not 

 been identified. 



Early work on Astrebla lappacea (Lindl.) Do- 

 min. indicated that it also has a period of dor- 



3 Lawlor, J., and Calder, D. M. Unpublished data on 

 germination of Stipa spp. and Themeda austraJis (R. 

 Br.) Stapf. 1971. 



